not all compressors have a clutch these days, some are permanently on and change their displacement according to load.

to properly test it you need to load it hard, i.e. have a hot (> 40deg.C day with full sunlight) environment, and measure the subcool out of the condenser and the superheat out of the evaporator as well as vent temperatures. obviously that is beyond the average DIYer and garage, so realistically it does just mean checking operation (vent temperature and demisting), but again in cold weather like this it is very easy to achieve low vent temperatures with a low refrigerant charge but it would struggle when it warms up.

why do you suspect it's not, or are you looking for advice when viewing to buy?