a/c recharge R 134a

any of you guys use the hose with out a gauge on the low side to add 134a? my son's 96 regal--(college car) is low-- prior owner said they add a can each year & it works all summer--
I do not have a gauge set-- & do not want to overcharge it -- but it is warming up in Kansas.

On another note is there a way to tell a system is charged with out gauges? such as how far the lines sweat?

thanks Lawrence