1. bounce the idea off of colleagues etc. Before anything else. Do they also think it's good?
2.This is the answer to a question which I should have been asking: specify what sort of chemist you're looking for, where they're located, what their skill sets should be and what their creds should be.
2. Be certain that their knowledge is current. Yours, too.
3, Find out where they're located. Much better to meet them face-to-face, even better to get the (honest) opinions of someone who you trust. Someone abroad who you don't know is likely a bad idea, for several reasons.
4. Make certain that they are also willing to walk-the walk, and not just talk-talk-talk. There are a lot of "experts" who are willing to sit behind a desk, but not stand at a lab bench.
Try posting these questions here?