You can post a concern about a certain site in this forum (https://bitcointalk.org) and see how other users think of a particular site and wait for a review. Also there is a certain site called badbitcoin.org where all ponzis are listed in their bad list.
But what if you encountered a newly made ponzi site and there's no review after all? Then you have to maximize your knowledge in the thing called "obvious". Earning without doing nothing in a site is totally a shit. Offering you a good returns in just a short period of time for let's say "double your shit within 24 or 48 hours" or something is really unbelievable. It's should be obvious why the hell your money will grow on a certain site just by depositing and will just wait.
Work to earn rather than relying on those shit site with shit returns promised.