You can never be too sure that the PTC site is paying and that it's not a scam, unless you experience it first hand. The best way to be closer to it is to carefully read their TOS (Terms of Service), FAQ (if they have one), and to periodicaly visit their forums. Note that forums are the best means to inform yourself as you can see: payment proofs, success stories, complaints, announcements... and you can comunicate with other members.
Many new members that sign up at a PTC site don't have a referral. Now what the PTC sites offer is a purchase of un-referred members. They usually come in packs of 5, 15, 35, 100 and 500 referrals. You can buy a pack that you see fit, and if the referrals are as active as you are, you will earn your money back in only 10 days.
Of course, if you can refer people yourself that's even better (the true meaning of free money). The more people you refer, the more money you will earn. Note that you can only earn money from your direct referrals.