Any line that runs from compressor to the condenser area or receiver at front of car is the highside. The evaporator line out of firewall and straight to either an accumulator or the compressor will be the low side. Line from condenser or receiver to firewall will be highside. The highside and lowside fittings are different sizes and for a reason, trying to charge into the highside can blow up the can of refrigerant and take off fingers or worse. I personally knew someone it happened to. Highside pressure can reach anywhere around 175+ (R12) to over 300 psi (R134A), the cans that refrigerant come in are only maybe 75 psi. You can see how that will be a problem.
You ALWAYS charge on the LOW side for safety reasons.
Usually two fittings only, a low and a high but an old car could easily have extra when someone alters something. The simple one-side-only charging hoses will always be equipped with a low side fitting hose. They do not even provide the ability to read highside at all as that is where the lawsuits will come from when an incompetent person blows up something in his face. Problem is, it is extremely difficult to know how much to charge using lowside only pressure readings. No self respecting A/C man would ever do that.
Not as simple as simply replacing the R12 with R134A, the oil in system is different, the o-ring material is as well. The receiver or accumulator whichever used in the system has a different type of desiccant (the actual drying agent) in it as well.