Screenshots, description and the reviews. If a particular game has reviews that are a bit mixed then I won't bother with it. Unfortunately, you can't rely on the screenshots much either because most people modify them
The reason I always try to get the reviews of friends is to ensure that I focus on doing what is right as regards getting the best from it. What I normally don't like about some reviews is that some may not be honest and you may end up buying or downloading a game that you don't like.
Well, I don't play a lot of different mobile games but whenever I want to download a new one I check the screenshots and the description first. After that I read some reviews and maybe also check for a video in YouTube to see actual gameplay.