My car has 75,000 miles on it.  I got my oil changed 10 days ago and have since noticed a leak from the car.  I thought it was the oil, thinking maybe they didn’t tighten things well.  Today I took the car back in to the oil change place and they told me it was the transmission fluid that was leaking.  When I was in their store previously (Danny’s Carwash)for the oil change, they tested the transmission fluid and told me it needed to be changed and would cost around $100.  They tested other stuff too.  I said no, didn’t want that service.  Someone told me that it’s possible they could have loosened the drain bolt because I declined service.  How would I be able to tell if the drain bolt had been loosened?  It just seems crazy to me that there would be a transmission problem with only 75,000 miles.  We live in AZ, though, so I don’t know if the hot weather would provoke more car problems.
 My dh and I know nothing about cars.
Frankly I doubt if anyone would intentionally loosen the drain plug. Possible, but not likely.
At 75,000 miles, I would have the automatic fluid changed regardless of its apparent condition. I don’t know about that car wash & oil change place, but while many are somewhat less than honest, I don’t think you have any specific reason to suspect anything dishonest now.
I doubt if you could prove anything.
If you no longer trust the car wash, then you should start looking for a trusted [b]independent[/b] mechanic to take care of your car.  Ask friends, neighbors, co-workers and family for suggestions.