First, as said above. Google them. Also ask them for a reference or two, if they are legit then they should be able to give you a ref. Also ask for other examples of their work. If they are legit they should have no problems with any of that.
Then, if you decide to go with them, pay 1/4 to get them started; 1/4 when they have a finished design and you have approved the wireframes,etc; 1/4 when they present working code for you and the final 1/4 when it goes live.
If you decide not to go with them well you already got some of his ideas so you are ahead of the game.
just my
.