Leak down is a good test to do as others have mentioned. If you can't get the compressor you might do a dry/wet compression test. It's a bit old school but effective. You look for the difference between the two not necessarily a number. So the dry test should register a number (use the screw in type) and that number across all 8 cylinders shouldn't vary by more than 10% between the high and low reading. Then put a few drops of
oil in each cylinder (just before you wet test that cylinder). The new reading shouldn't be more than 10% higher on that cylinder than the dry test.