Using your favorite search engine, search the following terms: “hosting company, review” or “hosting service reviews” and see how many sites come up. Check out the comments by users who are either happy or unhappy with their web hosting services. But bear in mind that most people who are happy don’t post reviews; usually it’s easier for unhappy clients to complain.
I usually glance through the complaints to see if there is any common thread. If I see a lot of complaints about tech support, such as “it took them three days to respond to my support ticket” I tend to note that. Slow support is a big issue when your website has problems. And complaints like “the server was down three days this month” would also be a red flag, because that’s a sign of an overloaded server or a service with poor support. Sometimes complaints are just a sign of a undereducated client, someone who doesn’t understand the process of maintaining a website, or keeping up with updates and patches to make sure their site doesn’t get hacked.