Your best bet is to have a quality portfolio build up. Then charging a deposit carries at least slightly less risk, because your clients already have a great idea of your capabilities, and if they didn't like your work, they wouldn't hire you. It's not so much about "good" and "bad" clients, as making sure you spend enough time talking things out with them before they sign anything at all. If you're both confident that you're thinking along the same lines, you really know what your client wants, and they like your past work, having a non-refundable deposit isn't a problem. Of course, you should make occasional exceptions, such as if the client hated the results because of something that was quite honestly your own fault, or if you realize you've taken on a project that's more than you can chew. Every rule has exceptions, and it's about knowing when to use them, and knowing when to stand your ground. A good balance, good work, and understanding your clients will be your best chance of not getting labeled as a "scammer."