It is very tough. You have to try to go to an event that they are catering this week. You are not an invited guest so you aren’t really supposed to be there. But that is the only way to really see how they operate. Are they always short-handed so there is a line for drinks. Are they always running around like they are disorganized. Are the guests smiling and enjoying the food.
Or you have to call their references, but you never can be sure if they are family members or truly “clients”.
usually it comes down to asking as many friends and family as you can.

I would look on yelp.com for reviews. I would also look at the menus and pricing after you narrowed it down to a cuisine. The factors that would make me select a specific business would be there customer service, once i narrowed it down to a few company’s, It would come down to how comfortable they made me feel, and ask for guarantees in quality, quantity, and timing. I work in the catering industry so if you needed some suggestions I could help with that.

I would look up ratings if possible. You could Google “reviews of _____” and see if anything comes up. I would also compare prices between companies.