Without taking the head and button off to actually look. Roll the engine over by hand, feel the pinch yet? Now do it again. Now mentally draw a pic in your head of whats going on in the engine, up down up down. Just split the difference in between the pinch and the point of least resistance. The piston will go full stroke per turn of the flywheel so when you feel the least resistance, that is most likely gonna be close to BDC. Or just take the cooling head and button off and actually look. Mark the flywheel with a sharp knife or a screw driver or sumthin, on the top of the flywheel. This way when your done playin, roll the motor over till the mark you made is where you know BDC is. Easy ain't it? I use a sharpie to mark mine because I take my motors apart and inspect them often, and the sharpie will eventually cook away leaving me with no mark on the flywheel anymore. And when the sharpie mark is gone, then I know its time to take the mill apart and check things out. Put it back together, check BDC, make my mark, and let it rip. Hope this helps you out.