how can I tell how much freon I need to add to an existing unit, without using a scale or sight glass

When the unit explodes then you know you've added too much. Why would you even guess about adding the correct amount of refrigerant without using the proper gauges?? This is highly dangerous to attempt otherwise. Don't do it.

Well you can use the feel method, big copper line beer can cold and sweeting, and the air out of the top good and warm. Second you can stick t thermometer in the supply right above the coil and see if you are getting about 40 to 44 degrees. 3rd put a amp meter on the power wire and see if it pulling the correct amps as stated on the tag. But the only real way to tell for sure is with the proper gages and a thermometer

Use the charging chart on the inside door of the system.