The only way is with a refrigerant identifier. Most shops that do A/C work should have one (in CA it's mandated). To just identify shouldn't run more than 1/2 hour of labor.

The valve thing is a good quick glance type check, but can be misleading as folks can put R-134a in without changing the valves. R-134a is used in many applications, only automotive has the different fittings. A/C techs from other industries have gauge sets that will connect to standard fittings, without the unique automotive quick connects.

If it's empty you can vacuum and charge with the refrigerant of your choice, if it's not empty you really need to know what's in it, as no one would want to recover what it does have and risk contaminating their equipment with unknown or mixed refrigerants.

MV