There are two petcocks on a Model T engine.
If oil doesn’t come out of the lower petcock when opened, there isn’t enough oil to operate the engine.
If oil comes out of the top petcock when opened, there is too much oil to operate the engine.
More importantly, I learned that both petcocks are unreachable and unseeable from where you add oil to the engine, and as an added bonus, I learned that any extra oil dribbles out of the upper petcock very slooooowly.
You could get sight glasses to install in the lower hole from period aftermarket suppliers. You still have to get under the car to see them. One risk is that if the glass breaks, you have an oil leak.
I was lucky enough to find a petcock with male pipe threads on both ends, so I made my own sight glass setup. I only open the petcock when I want to check the oil level, otherwise it stays closed. 
From my experience those gadgets are a constant oil leak and in my case I had to remove them.
