1. If they ask a lot of detailed questions, they are likely to be good. (What platform? Who is the customer? Anything working at present and so on)
2. If they tell you not to worry, and that all your problems will be solved, run.
Can you not tell from e.g. their portfolio or a short interview?
Any freelancing site that can figure out how to solve the ‘faker’ problem will be a massive success.