Simply by taking a look at the UX Designer's online portfolio, it's easy to get a sense of the standard of work on offer. You can then decide if it fits with your company's vision. It's also a good idea to verify that the work is in fact original. Normally there would be a "UX Design Berlin" reference note in the footer or elsewhere. Ask for contactable references, to get a sense of how the designer works with teams or alone.

