With the air conditioner running, the line that is cold is the low side. You put R134a in the low side. The high side is very hot, you might burn your fingers if you touch the high side line. Do not mess with the high side.
You need a pressure gauge to determine the air conditioner's state of charge. Walmart and Kmart have a recharging kit with a low side gauge for $25. Without this gauge, you risk the chance of over charging the system.