If it's bigger, then you'll need an in-person interview. When planning the total project budget, also allow for a post-programming meeting. At the least, that programmer will need to show you how the program works, and its internal design. You must have good communications with your programmer. No matter how skilled the programmer, if you can't understand each other, find someone else.
Lack of communications is by far the most common problem in getting custom programs.
If you have more than one project, then try to build a long-term relationship. Finding the right independent programmer is difficult, and you should try to avoid making the search twice. If you can commit to enough work over several months, you may be able to negotiate a discount billing rate. At the end of the project, the programmer should teach you about the work he completed for you. You may not master everything he says, but you can use it to improve your own programming skills.