A kitchen is a big investment, so you need to be confident in who you choose.
Check the reviews from customers. This will give you a great insight into the company.
Look at pictures of projects they have completed so you can see the work they do and not just the glossy brochure pictures from manufacturers.
You can also ask to visit previous customers to see the work in person. (We have lots of our previous customers who have offered to have potential customers come and see their kitchens as they love them so much!)