How do you make sure you're not stiffed? Do the websites themselves facilitate payment somehow? (Upwork, PeoplePerHour, etc.)
Any quick tips for where to look?
Try sending audio that has a watermark on it. It could be a recording of your voice saying your company/studio name every 5-10 seconds or so. This way, there's no way the client could use your work without paying for it but can still hear the track and evaluate it. Then tell them you'll send a clean, un-watermarked copy right after payment is sent.
This works better than sending a low-fi version of your track because I've heard stories of clients just using that and not paying for it. Also, are you working with a contract? If not, I strongly recommend you work under contract only and require a deposit upfront. This usually separates out the clients who are serious and trustworthy versus the ones who may be flakey.
Hope that helps!