Well here are some tips that should be able to help you make up your mind.
Does they have the skills to satisfy your needs? Are they more traditional or are they up to date with all the latest tools and technologies? Some designers are stuck in their ways using old techniques, that work but are dated. Some don't just specialise in website design they also do advertising material such as brochures. While this is good, you need to make sure your designer is up to date and can give you what you want in terms of website design.
If you have a deadline to get your site launched by make sure your designer can meet this and that they're not busy when it comes to go live. You need a designer who can be there when you need them, if changes need to be made they can do it then.
Have a look at your designers history as this is a good indication that they have the ability to meet your needs. Have they produced sites eCommerce sites for both trade and retail or have they only produced information sites. If you're working with a large agency make sure you who is working on your project and talk to them direct, so they understand your requirements.
Some designers have a particular style that may or may not suit your needs, whereas other may have a range of styles. The same as some might not have a large portfolio and other will have reams to show; make sure you pick one that will suit you best.
Website designers like others are proud to show off their work and should have testimonials from their clients, make sure you read these and maybe ask if you could speak to one yourself so you get your own opinion of their experience.
These are all important things to consider when choosing your website designer, or you can just choose us! Hopefully the above points have been helpful, if you do want to talk to us about being your website designer please get in touch Kat@e64.com