You will find a lot of meaningful information about some decent/legit companies here (on this forum). Second, once you've shortlisted a few companies, try initiating a chat with their customer care personnel. You will get a better idea about their legitimacy. This forum is full of such chats. Reviewing them can also be helpful.
More importantly, try googling the name of the company you want to choose with a few keywords (e.g., scam, complaints, dissatisfied customer, poor quality, etc.) that should return any negative/positive reviews. Do not, however, include "reviews" in the search because then you'll be presented with targeted traffic.
But, please keep in mind that even the best products/businesses in the world have dissatisfied customers. So, the point is gaining a holistic view about which one is legit and which not. Naturally, a legit company should have more satisfied than dissatisfied customers.
Once you're satisfied with your search, try placing a smaller order with that company and pay careful attention to everything: the email replies, the way they treat the factor of time, your order, the neatness/crudeness with which the paper is done, and most importantly the language and accuracy of the work. A responsible company is good at these areas.
Almost all that I have discussed above is already available on this forum: hardly any new information.
Since the flow of skills like website development, etc. has increased globally, most every website today presents quite attractive interfaces, layouts, and designs. So, it is now a thing of past to say that because the website looks great, they company must be genuine. It may not be so.
Hope this helps you some.