I would see his portfolio, and the testimonials.

Testimonials from big-guns speak tonnes about the designer's credibility.

If you are on a budget, check out Warrior Forum for some good designers. You can find some awesome designers out there who still haven't got into the limelight (probably that's the reason they are affordable to the masses)

There is no shortage of talent and professionalism on the WF.

Regards
Abhi