Not the easiest thing to do but it's all about the previous projects of a company. Just look at those, that's it
I don't mean to be the guy that responds with more questions, buuuuuuut.....
Before you start thinking about hiring a development company, have you validated your idea first?
Answer these initial questions and you'll get closer to what it is that you need so that development companies don't pad their quotes, you know exactly what you want first, and focus your seed investment around those objectives.
These should be covered before you go into a development company's reputation, fees, track record, and portfolio.
Look for good reference app developments similar to your field. Sounds basic, but be sure to be check how good your communications are particularly with the technical people not just the sales front. Ask to get phone call with a happy reference customer if you can. If they are over-promising beware.