As Scotty B said, some chords on the guitar are easily recognized and can be hint to what key you are in.
If not you can use relative pitch to label the chords by number. You can classify the progression as a 1 4 5 or a 1 6 4 5 or whatever you hear. Then when you get to your guitar or some other instrument you can use as a reference you can get the actual key.
Bob
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