Remove the glowplug and look into the hole, turn the flywheel by hand until the piston is at the bottom of it's stroke. Now take a permanent marker and mark the flywheel where it protrudes from the underneath of the chassis (some people scratch the flywheel so they always know, but I think this is unnecessary, after a while you get to know just by feeling where bdc is, so I use a permanent pen just so I can see it during break in).