I was a mechanic for 6 years and am ASE certified.
Typically what we would do at the dealership (I know, everyone hates dealerships) is when you bring your car to come get looked at, we charge something to look at it. To look at brakes we would charge something like $25. If you did in fact need brakes, we would apply that money towards the repair. If you didn't need anything at all, all you would pay is that small amount.
On a larger repair or diagnosis you may pay a good bit more to find a problem. Typically 1hr of shop rate time. If the labor rate is $75, you may pay a $75 diagnosis. Again, if you decide to fix the car there, some or all of that diagnosis fee should be applied to the repair.
As far as rotors being warped, some car manufacturers recommend turning your rotors everytime you get brakes put on. I worked for Honda, and we did not turn rotors everytime. If you do, you prematurly make your rotors too thin. You should know if your rotors are warped by if the car or steering wheel vibrates when you come to a stop.
You can differentiate between the front and rear rotors being warped by pulling the E-brake while driving down the road. The E-brake only operates the rear brakes, so if you pull it up while driving and the car shimmys, then you have a problem in the rear brakes. Pull the brake up slowly, don't go jerking it up.
If you do need to resurface your rotors, get them measured to make sure there's enough material to shave off. And you need a special machine to cut them straight.