Assuming the gauge is good:
Pull both plugs and turn the key off (or kill switch or both)
Install gauge in one hole
Hold throttle wide open while pulling
Take note of pressure, purge and repeat
Install gauge in other hole
Hold throttle wide open while pulling
Take note of pressure, purge and repeat
I then add some 2-stroke oil down the plug hole, enough to more than wet the top of the piston so it will wick down and lube the rings. Pull the motor over several times to get oil distributed.
Repeat above steps.
Compare any differences in compression which should give you an indication of how much loss you could be getting by the rings.
You are also interested in both cylinders being similar in compression.