How can I know which key a backing track is in?

Currently, there is no a better way other than using your ear to figure out which key it is in. 
However there are free web resources that claim that you can upload MP3s and the key will be detected. 
I haven't found one which does that with  100% accuracy though. 
If you know such resource you can drop a link on our forum

In general most Backing Track authors do state the key of their backing tracks and we're grateful for that. 

Rock on! 

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