How do I determine my window AC needs freon? It does cool but only part of a big room.?

Usually if you get a lot of ice buildup on the coils located just inside the unit it means you have a leak in it and probably needs Freon or a new AC. However, more than likely, your AC is undersized for the big room. It takes a very expensive window unit to cool a big room.

nope nope nope nope, simply get the garden hose and spray the dirt off of the fins on the unit, the fins are clogged with dirt, that is all that is wrong. the fins are the aluminum parts, very thin on the outside of the unit , kinda hard to miss. you need to spray directly at the fins, with unit off, you'll see dirty water start to drain out, just keep moving the spray up and down and all over til the water runs clear, then stop shut off water wait 20 min for all the water to drain out, then power up the AC and feel the cold air.

the AC may be too small to cool the entire room

Sounds like to A/C is too small. And, you can't re-charge them.