If your engine has been running without the distributor being disturbed, Yes you can take the cap off, see the rotor pointing at # 1, see the ball in the flywheel hole and know for certain that you are at TDC # 1 firing position..
If the distributor has been removed, all bets are off.. I usually spin the motor by hand watching the second valve back from the front. When the valve rises up ( # 1 intake closing ) keep spinning until you see the ball line up with the pointer, now you are at # 1 TDC. Drop your distributor in with the rotor pointing towards the # 1 wire position on the cap..You may have to adjust the timing, (move the distributor a little left or right) to get the engine to start..Works every time..
