You'll have to do some market research, dirty words I know in tech circles, but there it is =)
Have you personally, or your design staff anyways, ever played any free to play titles? And by played I mean having spent a couple months on both sides of the "I paid for some things" divide.
Find all the FtoP games you can, not limited to your genre but not ignoring your focus either. Categorize how they implement the microtrans in the game, how do the designs work, when you read their forums (and fan forums) feel stressful or simply involved in the game? Lots of work ahead of you since there are a lot of FtoP games lately.
Find the other free to play games that started life as traditional, monthly subscriptions and changed. There are some high profile success stories out there.
See if you can find information on conversion rates; I've heard that maybe 10% of players will buy something while maybe 1% will over-buy and be your backbone. I am not a reliable resource for information, however =)
In the end, without a living ecosystem (ie. a launched title) to experiment with you can't effectively test your game but you can research how similar titles have fared and maybe discover why, and adapt that information to your own use.