If it shifts down perfectly, but not up (smaller).
if it shifts down the same in every gear with no particular sweet spot(s).
if you can simulate a new spring by pulling on the parallelogram with a rubber band and get reliable shifting.
if taking the entire cable run out of the equation by putting the shifter right next to the dérailleur with just a foot of housing.
if the jockey pulley floats but not excessively.
you may want to change the spring.