If you turn the engine, you will notice that it gets harder as it make compression. I usually turn it both ways, (clock and counter clock wise) to feel for the soft spot. The middle of the soft spot should be bottom dead center. You can also pull the head off and mark the flywheel. Just pay attention to where you put the flywheel if you ever take it off.
ok, noop you say pull the head off, do you mean the heatsink head, am i correct and let me get this straight, i want the piston at the bottom of the engine and not the top when it is time for cooling?
I take the glowplug out. I take a hex wrench and carefully insert it into the glow plug hole.
I turn over the flywheel by hand until the wrench drops as far in as it is going to go before it starts travelling upward again. I mark the flywheel, and put the glow plug back.