There are so many different versions of the oil pans, and you haven't stated whether it has side kickouts, a lowered sump, or that it is a stock pan. There isn't an easy way that I know of. Kevco makes some oval track pans that have a sight plug at the top of where the oil should be.
In one of your other posts, you stated that you could watch the oil pressure to decide if you are low on oil. If you do that, you will be damaging the bearings and they won't live. Under power, you need to maintain roughly 10psi per 1000rpms.
I would suggest that this would be a good time to pull the engine, correct the oil pan gasket leak, add a dipstick or a pan that you can check the oil level, and change the bearings because they are probably already showing wear from the guy that you bought the car from.
Good luck,
Bill M