Assuming 2 strokes, even compression around 110-120psi and all cylinders within 10% on each motor, look at the plugs when you take them off and they should all look the same. Going for a ride if you can and making sure they do not hesitate and come up to full RPMs, 5500+/-. If you buy it with 2 stokes I always like to send the injectors out and do all the fuel filters, even the hard to get to ones, on a new to me boat.