First of all, be careful about hiring someone just because you like them. Many companies send their salesmen out, or their owner, and they may be very charismatic. But this does not necessarily mean that their work is high quality.
Be sure to check out the company. Ones who show you pictures of their work, let you meet some of the crew who will be on site, and owners who watch over the projects closely are all good signs of a quality contractor.
Online reviews are also helpful, but be careful about this one. Not all good reviews are real, and not all bad reviews are true. So take them with a grain of salt. A more important factor of an online review is not the review itself, but a company’s response to the review – in which you can learn a lot about the company.