So the first thing to do when you are checking or replacing the coolant in the air conditioning system is turn the fans to high, all the temperature settings on the coldest and turn it on to the air conditioning setting. So once you are under the hood of the car figure out where your low and high pressure lines are for your air conditioning system and only use the low and what you are going to do is get a gage and get the air conditioning you want and you clamp the gage on or some screw on. Once you have the cap put on the line it will have a gage and it will say low, filled, or alert and so under 25 pounds it needs to be filled. Anywhere between 25 and 45 means it is fine but over 45 pounds it has too much pressure which means you might have a problem with the system or it is over filled and if that is the case you need to drain it. The tube you are looking for is usually between the compressor which is on the engine and the evaporator which would be the part which is on your dash or on the firewall and it depends where it is, on some cars it is on the driver's side and some the passenger's side so check with the owner's manual and that is the best way to figure out where it would be.