Bring the #1 cylinder (the cylinder closest forward to the radiator) to top dead center. I used to use a metal wire to feel though the spark plug hole, when the piston was coming up to TDC.
Then take the cover off of the distributor and see which cylinder on the rotor it points to.
Be careful though and get the piston to the top during the compression phase (both valves closed) and not during the exhaust phase (exhaust valve open). Otherwise you will wind up marking the cylinder 180* out.