Sometimes on the ticket it is not clearly mentioned whether the flight supports Wi-Fi. Is it safe to turn on the Wi-Fi on electronic devices whilst on board to see if the flight supports it?
There is no single source to check if there is Wi-Fi in a certain flight or not. The best thing you can do right now is to check the airline's website to see if the flight supports Wi-Fi or not. Remember, if the airline advertises Wi-Fi that does not mean all its fleet/flights has it. Sometimes certain types of airplanes in an airline have it while others don't, same goes for routes, like in American Airlines where Wi-Fi is supported within the US only! international flights with the same airline will be without Wi-Fi.
Alternatively, use google to search using a query like
Regarding safety of the Wifi, it is safe to turn on. Check my answer to other similar question, it will answer your question.
In general airlines will go out of their way to tell you if the plane you are currently on has Wifi/Internet Access - as that's how they make their money out of wifi! How they do this varies between airlines, but generally includes :
Given your question was actually is it safe to turn on your Wifi, then obviously if if any way you have been given an indication that the flight has Wifi, it is safe to turn your computer one (once portable electronic devices are allowed).
If no mention has been made by the crew as to whether the flight has Wifi, and if there are no indications, then this would normally mean that there is no Wifi. As a general rule, if a plane does not have Wifi, then enabling Wifi on your computer (which thus makes it a transmitting device) is not allowed, as the plane will potentially not have been officially certified that Wifi devices do not interfere with it's systems.
However realistically, turning on your Wifi device is not going to cause any issues. The FAA and the airlines are aware that most people do not disable Wifi on their computers/tablets/etc when they are used on board (more through ignorance than anything else), and as yet there have been zero confirmed cases of Wifi interfering with the systems on board an airliner.
So if you need to know if your plane has Wifi, the two best options would be :
Officially the first is the correct answer - but either will work...
You can check seatguru ( example). Under "In-flight amenities" wifi availability is indicated. Some airlines are still experimenting with only a handful of planes with wifi, but I expect seatguru to be the most complete resource on this topic.