Hi Tin,
Will keep this Drupal specific since there's loads of really helpful guides on selecting a generic web developer. These are not recommendations of particular devs, just general guidelines.
Drupal Version- Which one is it?
This matters a
lot of late because the latest version (version 8) significantly changes the way Drupal sites are built. Drupal developers are still catching up to this change. Therefore, if it's a new site it's best to start on Drupal 8, but be aware that an understanding of Drupal 8 is needed. It's sounds like it's an existing deployment though. Is it version 7?
How active are they
Like any framework/cms, the more time you spend with it the quicker you become at solving tasks. In the Drupal world this might be:
- Which modules are you most familiar with?
- Are you familiar with the rules module? (Note this isn't ready for Drupal8 yet)
- Do they have a Drupal.org profile, do they commit code back or discussions?
- With non-Drupal developers, there's a tendency to jump into writing code when actually there's already a module which will do that specific task (There's a chaper on this in the book "The Definitive Guide to Drupal 7") and is still relevant to Drupal 8.
Type of work- theming or logic
More so in Drupal 7, a theme developer requires a lot of experience. The theme system in Drupal 7 cannot be described as simple. Therefore people tend to specialise as Drupal themers, and let others focus on the logic. You might find it better to focus your search on a team of people who can cover both programming logic (aka "back-end") and theming.
Whilst I've no data on this, Drupal seems to be the tool of high budget projects (define high?) this may be due to the complexity and planning required when building a Drupal site; the same can be said for any development. The planning is important, not always the chosen platform. I raise this because unless you find a very seasoned freelancer, it's unlikely they'll be experienced in both themes
and development.
Finally, Drupal is about community it has the saying "come for the code, stay for the community". Don't be alarmed if the developer(s) you find are always engaging in forums on Drupal, asking questions to find out how to solve things you ask of them- this is a good sign that they're checking to see if the community has already solved it (and usually it has) rather than re-inventing the wheel.
Hope that's useful
