This is inspired from this answer to my question - Can there be malicious apps on Google Play?
Aside of that: In case you're still unsure, pick a good forum and ask. Another good idea is to check other apps from the same developer (just follow the link on his name), and use above criteria on them.
Since currently finding an ad-free application is not easy, is it OK to ask for recommendations (i.e. for example "Can someone recommend an ad-free FTP server that is not a virus")?.
I looked in the FAQ and it said app-recommendations are considered "primarily opinion-based", and if I ask "What is the best app doing X", it would be true, but ad-free and not-virus is just true or false, there is no opinion there.