Leak. We know its on the inside because we checked the outside. How can we locate the leak, My

Question

We know its on the inside because we checked the outside. How can we locate the leak, My husbands income is the only thing we have coming in right now due to me being layed off.If we can't do it than give me a ball park of what it will cost, we had some one tell us they charge 65 hour I can't afford that. Just a ball park if you could to see what we could come up with just in case this person has to come out. Also how much does a coil run for this unit.