My ac is running but no cold air. How do I know that the freon is out?

Is your filter good? The big copper line should be sweeting, the little one should be hot, and the ac should be blowing hot air out of the top.

When it's out, the low pressure switch won't let it kick on.

Simple performance test is usually the temperature drop across the coil. It is a function of humidity, but 15-20 deg is a benchmark.

Low freon is not the only cause.

Leaks should be repaired.