Couple of different ways to do this

1) You control the process from start to
finish.

2) Put your tracking vehicles in place as sales are made
wither it is a website or a phone number

3) What i normally do is have a clause in the agreement
that states i can at their expense have my accountant
look at the books as it pertains to this project only.

If a guy is going to rip you off quite frankly, he is going
to rip you off.