1. Is the compressor clutch engaged? You can tell this by seeing if the front "hub" or drive plate is drawn in when the system is turned on. You will usually hear a "clicking" noise when this happens. If it is engaged, you need to shut it off and put a set of gauges on the system. If you have only a low side gauge that comes with the DIY charge kits, use that. It is better than nothing. Normal readings should be in the 30 psig on the low side and 180 psig on the high side range. These are approximations as each system is different so you will have to look  up the pressures for your particular vehicle. What matters is that the low side is close to 30 psig and that the high side is not outrageously high or low. The low side dictates the degree of cold. The lower the low side reading, the colder your system will be. Too low, and your evaporator will freeze and stop blowing cold after a while. Optimally, you want a low side in the high 20s to low 30s.

If you do not have gauges, feel the evaporator suction pipe. It is connected to the largest hose in the system, which runs from the suction side of the compressor to the evaporator. If it is cold, that means your evaporator is cold. If you have warm air coming out your vents and a cold evaporator, the problem is most probably in your vent system in your dashboard. Check all your vacuum lines (many of the switches are controlled by vacuum), and look for stuck damper doors that control the flow of hot and cold air and the degree to which it is mixed. A door stuck in the wrong position will prevent the cold air from reaching the vents.

Look for the source of the leak. This maybe as simple as looking for a cut hose or as difficult as finding an intermittent leak in a compressor seal. The sign of a leak is the oil spot associated with it. Hoses will typically have wet spots where they are leaking. Look where the rubber meets the crimp. If the rubber is bubbled, excessively oily or cracked, it needs repair or replacement.

A leaking compressor will usually have an inordinate amount of oil at the point from which it is leaking. A worn shaft seal will usually leave an oil stripe on the hood directly above the pulley.

Check your evaporator case for oil. The evaporator is the coil inside your dash that gets cold. Run your fingers along the bottom of the case feeling for oil and, if possible, look or feel inside. An oily evaporator is a leaking evaporator and conversely, a dry evaporator does not mean there is no leak. Ideally, you want to put an electronic leak detector in the dash vents to check for the presence of refrigerant to confirm your diagnosis. Unless your leak is large and the quantity of oil substantial, it is worth paying someone with a leak detector to confirm your findings. Usually, changing an evaporator is a big job.

Check your condenser for oil spots. The condenser is the coil in front of your radiator. Look at the center and at the ends. Look for kinked or dented tubes. Look for shiny spots where the metal may have been nicked. Again, oil spots mean it is leaking. Although not as difficult a job as an evaporator, this diagnosis may also be confirmed with a leak detector.

You will need a Refrigeration Manifold Gauge Set to properly diagnose your system. We will use 30 psig on the low side and 180 psig on the high side as a norm for the purposes of this discussion