in my opinion you cant know it for sure before the first payday.
but you can minimize the risk of being scamed by:
- choose a campaign managed by a member that did it before and received positive feedback
- try to find out if the project that is advertised in the signature is legit or not.
- dont choose sig of projects that look fishy.

e.g
i know that byteball is not a scam and i know that the campaign manager managed a lot of campaigns before and kept promise.