Who pays them?
Most brokers are paid a commission by the funding partner. That is normal, but they should say so up front, and tell you whether you pay any broker fee on top. Ask for the answer in writing.
Will they show the total cost in dollars?
Ask for the total payback, the cost of funds, the net amount deposited after fees and the payment, all in dollars. A broker who talks only about speed or the size of the advance is leaving out the part that matters.
Will they tell you when not to borrow?
If a bank can fund you in time, it usually costs less. A broker worth using says so. Be wary of anyone who treats every business as a fit.
What will they not promise?
Nobody can promise approval, a rate or a funding date before a funding partner has seen your statements. Treat a promise of any of these as a warning sign.
How do they handle your information?
Ask who will see your details, how long they keep them, and whether you will be contacted by more than one company. You should be told before you agree.