Nearly every specification chart for a speaker you’ll find will list that speaker’s sensitivity in decibels. For the most part, sensitivity tells you how much sound pressure level (SPL) a speaker can produce from a 1 watt signal, at a distance of 1 meter. For example, a Celestion G12M-65 Creamback speaker has a sensitivity of 97dB, meaning that it can produce 97dB of volume from a 1-watt signal, measured from one meter away.
The “for the most part” caveat above is that frequency response also plays a role in sensitivity. As frequency response drops off at the low end and high end, so does sensitivity. So as you get out to the lowest frequencies and the highest frequencies a speaker can reproduce, the sensitivity drops accordingly.