Well here are some tips which will help you to understand a site's admin mind. He/she is trying to scam or he/she is a trusted person!
1. First of all, whenever you visit any new ptc site. Look at its template and its words written there.
Template: Template should be professional!
Words: There should be not any mistake in the words spelling. Well any mistake in words seem like admin just want to get money after making this site!
More tips.. Stay tuned! :)