The best way to judge whether a designer is suited to your project is to have a look through their portfolio. Design work can be very much in the eye of the beholder, so you need to find someone who has a style you like.

Unfortunately, this won't tell you what they're like to work with so maybe ask for a reference. Setting a series of project deadlines / goals upfront should also help.

It can also be tricky to work on a brief over email if you've not met the client in person. I find it's always a good idea to speak to any remote clients on the telephone as that gives them the opportunity to talk you through their ideas and brief. A proper conversation can bring up questions and ideas that wouldn't necessarily come up in an email exchange.

In the past, I've found it hard to find good developers. If you're not a developer, it's difficult to tell whether someone is good just by looking at the sites they've worked on. I've come across several developers who talked a good game but couldn't deliver before now. The agency I used to work at for example, hired a few developers who weren't up to the job. At least with design work, you can judge the visual aspects.

Links to my portfolio and company sites are in my profile if you're interested in working with me.