Yes, a compression test is definitely a good thing to have as an evaluation of a used car. A leakdown test at teh same time is a good test too. Checking the coolant for signs of cross contamination is inconclusive because anyone worth their salt that knew they had a bad headgasket would change the fluids before putting the car on the market, so fluids will always show good.
Other good tests:
Cooling system pressure test.
CRITICAL: complete and thorough visual examination of the brake system on a rack for evidence of fluid seepage, corrosion, outright leaks, stuck calipers/cylinders, and parking brake function. Low pads would be a negotiable item bu not a dealbreaker. Loss of brakes can kill you. Don’t overlook this.
A complete and thorough examination of the chassis as well as anomolous wear patterns on the tires.
In a framed vehicle, like a pickup, a thorough examination of the frame for rot. In a unibody, a thorough examination of the underbody for rot.
A check of the lights, wipers, etc. All these things functions should be “run through their paces”. Wiring problem can be a bear to get stuck with.
Evidence of a past accident. A neighbor bragged about the great deal he got on his used car and found out a month later that the rear of the vehicle was pop-riveted on…and not very well at that.
A good road test including highway driving. Push the car on the highway and see what happens.