There are a couple of "feel" checks that the experienced 1/2A engine users do that take 30 seconds. Crank fit and cylinder leakage.
A really well fit engine will allow the piston to bounce back and forth between TDC and BDC when you smack the prop. There won't be any axial play as you forcefully turn the prop. The cylinder should remain pumped up for at least 3 seconds after you bring it up to TDC, hopefully much longer.
A soda straw, can of water, duct tape and your breathe are all you need to quickly and definitively chase down crankcase air leakages.