Wifi-equipped aircraft have a logo on the fuselage that you will see as you get to the end of the jetway and prepare to cross the Sacred Threshold into the cabin. It's an antenna-like graphic with the caption "Wifi-equipped aircraft" or something similar. And, the FA's will include this information in their announcements. One way or another - you'll know it! Other than asking at the departure gate, I am not sure that you can divine this information much earlier. However, others may know differently and if so, I am sure they will chime in here.