Another method I like is to use a clear hose that fits tightly in the glow plug hole. Use #4 as its TDC is the same as #1 but it is easier to access. Seal around the hose with a bit of grease. Put the other end of the hose in water. Turn the crank one direction. Note whether bubbles come out or water goes into the tube. If water is being drawn in, then rotate the crank the other direction. Go very slowly and watch for the moment the air stops being pushed out and instead begins being drawn into the tube. That precise moment the crank is at TDC for #1 and #4.