In a healthy system the smaller pipe ' sweats'... If it starts to freeze, then you need more gas.
Actually, that would be the LARGER pipe, the vapor return line. If it ices up, that is a sure indication of low refrigerant.
Also, the small (liquid) line should get hot. If it doesn't, that is a sure indication there is little refrigerant in the system.
Many systems have low-side and high-side safety controls that shut it off if either the high-side pressure goes too high, or the low-side goes too low.
Some systems have these latch, so you need to press a button to reset them. Seems like this system does not have that latching feature.
Some systems may also have a sensor to detect iced-up evaporator coils, blocked air filters or broken fan belts. That would be the air handler unit.
Jon