Look on the compressor. Theres a spot for cap size but half the time it's filled in with zero's.
Next try to google the compressor model number, or give it to your supply house and have them look it up.
One word of caution, if this unit is old enough the original size might not be easily available and the replacement compressor cap doesn't always match the original.
I have an HS25-411 at my house, had the same problem when I got it off a job 11 years ago. The cap for your unit is somewhere between 30 and 50, find the one that gives the lowest start winding amps when running. The start winding amps should be 3 to 5 amps lower than the common and run. Say you have 10 on the common and 6 on the start 9 on the run, it should be ok. If they all read 1 or two amps different the cap is too big.