Well, if so, here are a few things that you may want to consider:
1 - Does the designer have a portfolio of previous projects to show you ? This is a great starting place to see if you feel this person has the ability to work with you. I would suggest that you look for a range of projects that may or may not fit your personal aesthetic. Someone that is diversified and able to meet the needs of a range of clients, is more likely to meet with you and strive towards something that is personal to you. If I am looking through a portfolio and everything looks the same, then I am inclined to think that I am going to end up with the designers aesthetic, not my own.
2 - I would want at least two references from recent clients who's projects are complete.
A quick call to see if they are happy with the result. Did the designer listen to their wants and needs and did they find solutions that suited them. Did they find the designer to be knowledgeable in his/her chosen field. Did they prevent mistakes from happening, before they happened. Do they feel that they received good value for their money? Would they hire that person again and would they recommend that person to a friend, family or colleague ? If you can only ask one question, maybe this would be the most telling.
3 - How does the fee structure work? Of all of the clients that I have talked to over the years, this is the one issue that leaves most clients upset at the end of a project. The potential for unknown/hidden fees that often come with this sort of arrangement can leave clients feeling like something is awry . Generally, you should expect to pay an hourly rate as well as "actual cost" plus a percentage on purchases. This is the fairest way to both the client and the designer and it insures that the designer is making decisions for you based on what is in your best interest, as opposed to their best interest. Very often there are back room deals going on whereby a designer may receive a "kickback" from a vendor for bringing their clients to said vendor. If you are paying an hourly fee for advice and that advice is possibly being swayed by the amount of a kickback from one vendor versus another, well, that is unethical and you are bound to end up with some bad advice. If you are made aware that this is the arrangement, then that is a different story.
The advantage to the hourly rate and cost plus method is that the client may get to take advantage of pre-negotiated discounts, which can help to off-set the cost of a designer and this also means that you don't have to go in and bargain for deals.
Happy hunting !!