Well typically you'd look for the CC (cubic centimeters) of air it pumps out per minute...
500 CC's/min per gallon is the common accepted value...
Check the bottom of the pump for this value... If you cant find it there, check the pump manual, and it you dont have that, search for it on Google by using the brand and model and putting CC at the end of the sting...
for example: Elite 801 CC
This would search for a page that has an Elite 801 air pump, with CC somewhere on the page...
If CC is on the page, its more than likely talking about the CC/min...
Use my example above on Yahoo! and you'd see the first link to come up shows the CC/min value of that pump...