leak down testing/tester;

put each piston at their tdc on the firing stroke and screw the fitting in the sprakplug hole with the piston at tdc

supply 5 psi or so, air pressure and listen at the carb, exhaust, radiator, oil fill or dipstick holes. listening for any air coming out and bubbles in the radiator. Also if the kit has a gauge then watch for pressure drop in a given time span, should be noted in the directions with the kit.