Pro tip: I tried a compression test before buying a 1966 Mustang at a swap in Dallas, and it saved me $4,500
The seller wanted $11,000 for a car that looked great, but the test showed two cylinders way low. He dropped to $6,500 once I showed him the gauge numbers, and I walked away anyway. Now I keep a tester in my truck at every show, so what do you check first on a car you can't drive?