This is 2 years old, but still quite relevant and as the answers are somewhat lacking (in my opinion), I'll share my experience and solution to the question for anyone who might be or still be looking for some tips and answers.
The first thing about mobile (Android especially) is that there's no flat ground nor standards, regardless of what anyone tells you. iOS makes things a bit easier because there's only 1 source (Apple) and they usually follow their own set of rules, but even iOS gets modified now and then in a way that break things.
As such, it's always best to not get too deep into the search for the perfect solution as any solution will become usually obsolete at some point in time, be it from an update to Unity (the engine) or the newest generation of Android or iOS released and applied/used by the manufacturers.
Still, it's normal to wish for a good approach and one of such would be to use 2 methods that works hands in hands and to which Unity has full support (or at least offer good access to).
The first method is to use some hard coded system stats.
This is a good way to get an "idea" of the average memory available for the app. The one thing to remember is that, on mobile, this will NOT return the full value of the available memory of the device, but will return what memory the OS allows the App to use when it's being used in Foreground. For example, if a mobile device has 1.5GB of RAM and the version of Android uses 1GB of RAM to run in average, systemMemorySize will return around 500-512MB. Due to that, it's not a "work-best-and-always" trick, but it does help you know how much memory the user's device might have in general. (For example, even if a mobile device with 4GB of RAM, if the user is leaving the Facebook App, a web browser with a few tabs, chat via Skype and play the game at the same time, the game might be able to get access to only 0.5GB-1GB memory. Some app used by many people every day are real memory hungry and there's nothing you can do about.)
Keeping track of the FPS within the game can also be a good gauge.
By using a coroutine, you can check the FPS every now and then. There's no need to check it every frame though. The trick is to check the average within an acceptable amount of time and drop a few things if the FPS average gets under a certain threshold. But this is assuming that you have already profiled your game so that it remains constant and fixed in its FPS at all time. If you know some part of your game might have some heavy impact on the FPS and wishes to be safe during those moments from having fake drop in the FPS, you can always set the threshold on the fly as needed.
Another thing that can be useful is this function:
While the example in the link is useful, one good example of use of the lowMemory function is to do a memory sweep and load a "safe scene" which allow the memory usage of the App to drop. An important point to remember is that, whenever lowMemory is being called, it's not that the FPS is just low, but it's that the OS struggle to maintain the App integrity and might close it soon if nothing is done.
A last thing would be to consider checking the version of Android or iOS that is being used. There's a few way of looking it up (like this ). Knowing the version won't give you a clear answer, but the API version (OS version) gives an idea of what kind of compatible or incompatible thing comes around from the OS itself. You might be able or forced to turn off some features or graphics effects if the device runs on an API below 23, for example, like AR/VR or physics-based particles. If a device API is 20 or below, you should consider, by default, that it's a low-end device (in today's and Unity's mobile standards) 23 is low-average with compatibility with enough memory. 24 to 27 is low-medium-high and are the most used in the World right now. 28 or more are high and should, generally speaking, run well unless systemMemorySize returns a low value.
Remember that "high" models, in what I wrote above, are NOT the best available, but more like the best available in a big part of the World. When it comes to testing your App with multiple devices, it's best to do the simple, yet most common thing:
Go to a popular store around your area and speak with the store clerk. Explains your position and ask him what's the worse phone (or tablet) he has in his opinion, which one is the most popular outside of the newest models and, if your budget allows you to, which one is the most popular (which is usually one expensive). You might know more about mobile device than him or her, but he's the one who actually sell the devices and you might be surprised about what models will comes to your hands.
If you explain your position as a mobile game app developer, not only the store clerk might be interested in your own story (and game), but he or she might be more willing to help you with honesty. (And he or she knows you might return for another testing device in the future.) Obviously, some store clerk might attempt to mislead you in one of the devices, but remember that if he tries to mislead you, he'll mislead others and that's a common thing which makes it so that your "average every-day's Joe" will usually buy that thing because the store clerk sells it to him (or her).
Also, last tip, don't purchase device below your minimal standard just for the sake of, maybe, be able to downgrade your project below your expectations. If you set your goal for the App to run on devices with at least 1GB of RAM, don't buy devices with 512MB of RAM. Remember that both the Google Play store and iOS iTunes store (for games/app) allow you to set a "minimum" level of compatibility which will make your App visible not compatible and downloadable with device below your target. Don't fall into the rabbit hole of over-optimization for the sake of reaching a lower device as this will often result in breaking something in the app for higher/better devices.