If you aren't having any operational problems and don't suspect air or moisture contamination, you can do a simple smell test.With vehicle engine off you can either depress the low side service valve slightly just to get a smell.A charging hose may be necessary to do this as long as it has a valve connected so that you can control the refrigerant flow.All you want is a small vapor sample.Please don't blow the charge or try to freeze your nostrils.Use common sense.A healthy system should have almost no to very little odor.A burn't oil or acidic smell may indicate the presence of air and moisture in the system and should receive further attention.A neutral to slightly sweet smell is normal.