how do I know a site really pay?

If you have trusted friends who can show you proofs of payment from the sites they have joined, then you can be sure the site is legit. Just keep yourself updated on the status of the sites you've joined and keep in touch with other members, too.

It is really hard to know if it is a scam or not. I do reseach on some but not all. I pretty much wait for users in here to give their feedback and whether they have been paid by a site or not. I give them a chance since they are free, and I have no money invested. I do them for awhile until I see which way they will go where they are will be legit or go scam. They are easy enough to do, so at least trying them it is worth it to me. Sure, they don't pay all that well, but it all adds up in the end, if a person does enough of them. Good luck to you and happy mylotting.