Testing battery?

Okay so I have a new-ish Powershot S80. I just got two replacement batteries from SterlingTek.com. On the site they are listed as 1100mAh, but on the battery it says 1500mAh. Either way, what would be the best way to determine how much longer this one lasts in comparison to the original?

Best way would be to use the camera with the new battery in it. Remember it does take a few charges and uses to get the battery to optimum level, so you might have to use it for a week and then compare the time it lasts compared to original battery.