A few ways.
1) All bidders must provide a valid Paypal account before placing a bid - that is, a valid Paypal ID where they will accept payment at job's end - prior to placing a bid on your job. Furthermore, if they have a silver shield icon next to their name on the bids screen, their Paypal account has been verified with their CS registration email.
2) Feedback, feedback and feedback. If the bider has prior jobs under his or her belt, they are likely to complete your job. If it's an important task, only choose Providers with a good feedback rating. If it's not such a critical chore, save some money and pick a low bid from someone without much feedback.
3) We keep track of non-performing bidders and only give them 1 or 2 chances.
4) Watch for our upcoming guarantee. After we launch that, if a Service Provider fails to complete a job we will pay you for your trouble. You can tell if the bidder is guaranteed because they will have a Guaranteed Badge next to their name. We can't wait to launch that!