Plan on using flight time to catch up on forgotten emails? Avoid the heartache of stepping onto an airplane only to discover it's WiFi-less by checking the online status of your plane in advance.
If you want to purchase a ticket based on which airline has WiFi onboard, then note it during your search. Travel websites like Hipmunk and Kayak include in their results details whether a flight is WiFi-enabled. When choosing a ticket solely on this feature, though, do realize that technical difficulties are always a possibility, and despite the promise of Internet, there may be an off chance that the wireless connection is down.
When you're just curious of whether your plane will have WiFi, turn to SeatGuru for a breakdown of every airline's fleet of planes and which have WiFi. Cross-reference that with the plane information in your travel confirmation details to know that, yes, your American Airlines flight on a Boeing 737-800 does have WiFi capabilities.