The only way to know is put some pressure gauges on it. Now adays you can buy a can of 134 with the little gauge already on it and add only what you need. It even tells you when it's a good charge. Try it.
Put on LOW SIDE ONLY, it will have a blue cap on it, unsrew the blue cap and you will find a little quick coupler type fitting. You might need to jump the a/c compressor to add it if it is too low of pressure.

Dean