I'm thinking about making an indie game and want to know if there's maybe one with similar idea.
I'm not asking about my idea specifically, I just want to know how to find out if a game has the same idea as the one I want to implement.
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I would like to add my own answer, which had elements already propositioned by the other two users here. However, their suggestion isn't the same as mine.
As Patrick said, "There is no shortcut to experience" and Byte56 said, "It's a research task." Both are great answers, and both define quite well what I was going to say before reading them.
Find someone (or multiple someones) who have experience. A lot of experience. A priority would be experience in the genre and platform, but a secondary priority is always beneficial when it comes to experience in gaming period.
For example, I have been playing PC games for over 15 years, and for many years I would play 18 hours a day some days. Not only this, but I am a renaissance gamer who has played nearly all genres, excel at all of them and typically come out on top in competitive play no matter the game, have read books and endless numbers of articles on game design, and I'm a game developer. There are thousands of people like me on the internet, and they are often very easy to find because they don't just love to play games, they love to talk about them, love to boast of their experience, love to be vocal on forums. (Ex. you could find a multitude of people with MMO experience, dedicated, decades of experience across endless numbers of titles, on popular MMO forums. They would be easy to find when browsing these forums, and would be VERY likely to respond when polling them about their favorite genre.)
The reason I suggest asking someone with a ridiculous amount of experience is that people like me have lived and breathed gaming quite hardcore, often for their entire conscious existence. As anyone who is devoted or in love with a specific hobby, they will be exuberantly available to answer all sorts of questions. Example, if you are making a MOBA game, then ask gamers who are passionate about the MOBA genre. They will be very willing to share, and to share in detail. They will have the experience to know not only one game, but most likely every single MOBA game. Even the ones no one has ever heard of!
The first priority takes precedence of course, as for example someone like me, would be ignorant and incompetent if asked about Console gaming. You'd probably get an incredibly biased, erroneous opinion along with a rant about how their genre (ex. PC gaming) is better than the other platform (ex. console gaming) in every way. This would be useless and a waste of time. Someone devoted to one platform who despises the other would be blind to the benefits of experience of other platform games similar to yours.
If you cannot find multiple people who are both hardcore devoted gamers and developers, then you should find some of both. Not game developers, but people invested in game design theory. There are plenty of gamers who don't know why the like one game but not another similar title. There are plenty of game developers who don't even think their ideas through before implementing them. However, the more experience one has, the most likely they've thought in depth about their hobby or theories before.
As Patrick suggests, nothing beats experience. Those who have significant experience about professional opinions, books, articles, or blogs, will not have their own thoughts, but will have a collection of understanding of game design theory from everything they've ever read. This can include thoughts from extremely experienced "master" game developers such as those who are part of "the greats".
When you lack the experience, get the thoughts and opinions from those WITH experience. I myself love to read what players think when they have been gaming for as long as me. However, it is difficult to take people seriously when they talk about how they have only played a very specific genre, only a handful of games, for a meager amount of time, but think they will revolutionize the genre with their game idea.
Part of what I find to be true in ANY field is this: Those who make quality products are those who know about the product firsthand, OR those who hire or outsource knowledge about the product firsthand. Most people who try to make a product (ex1. a business man who tries to make a AAA next-gen video game but doesn't know how to turn on their console without tech support) produce a horrid product and then are confused as to why their project failed. Another example are game design theorists or newbie game developer wanna-be's who post on forums "I have a really good idea..." and the contents within have horrendous flaws that anyone who plays the type of game could see immediately. It is not the scope of their project that results in facepalms, but their lack of experience in game design or the experience in design theory.
With all that said- finding experienced gamers, and you will immediately get a long list of similar games. A list that would include games that no one would think are similar, but due to a specific feature are in fact similar. Not only this, but in-depth details as to what similarities the games have to yours, how the features played out in the other games, and good/bad reviews of the features within each game on the list.
Why go out and do research on your own using something as vague and crap-friendly as google, when you have a massive network of human beings with decades of experience and tens of thousands of hours devoted to playing games? It's not a hard task for them, because they have ALREADY finished the research, and you can find enough of them to get a basic understanding as to what is a common complaint or praise to your game's features, based on their collective experiences.
It's a research task. You need to define your game well enough to assign a genre to it. Then find games in the same genre. Watch play videos, read descriptions, buy and play the games, etc.
(Further, you can post about your game online somewhere, and you can be sure that someone will question why you're making a clone of game X.)
However, as Code Assassin said, you should just try to make the game you want to make. It doesn't matter what someone else has made. Playtesting your own game should give you ideas on how to improve the features you've implemented.
There is no shortcut to experience.
For one directed avenue to achieve this experience check out David Perry's Game Challenge
You may also start working on your google skills to track down genre reviews and then read more deeply into titles that sound interesting. There is a drawback to this in that by only searching for genre titles you'll miss out on good ideas from outside, I think that cross-pollination is important to keep things fresh.
And don't forget about non-computer games, board games and dice have been around since before the ancient Egyptians.
It all comes down to playing a lot of games, reading a lot of reviews, following game forums, and it's a lot of time and work.
In your case, so this doesn't take a lifetime, I'd start with just one day of google and try many different searches (for example: "2d platformer with jumping") to get a broad overview. Then on day two only read the small number that you found interesting.
Then, like the other answers will most likely tell you, go do what you want =) and let testing help you decide what's a fun combination of features for your vision.