You can simply take the intake valve adjustment cover off and turn the engine (engine turns same direction as your front wheel when riding the bike) til you see the intake valves open and then close. Next, feel the piston with a long phillips scredriver and slowly turn the engine til the piston comes completely to the top. Usually one is adjusting the valves when TDC is needed so taking the cover off is no big deal, but if you had to find TDC compression for whatever reason and didn't want to remove the valve inspection cover, you can hear the engine sucking through the intake. Old 4 stroke guys always listen to the intake to find TDC compression when starting the old beasts.
dan