For what car?? The evaporator is usually buried in the dash. Somewhere forward of the glovebox or the radio. Not the easiest thing to get to...

There are 2 valve stems 'somewhere' in the refrigerant lines. They look kinda like tire valves, only bigger. One's high & one's low pressure. Use them both when you're measuring operating pressures, but for adding fluid you gotta know which one's the low one.

Sometimes they're marked. The high-pressure line goes from the compressor outlet to the condensor (in front of the radiator). The low pressure line goes from the firewall to the suction side of the compressor.