Like Tattie Bogle said. You look at the key signature if it's written, or count the sharps or
flats if it's not. You do have to learn that 1 flat is F, 2 is Bb, 3 is Eb etc. though...and you have to learn that Dm is the relative minor to F, Gm to Bb, etc.

Mostly, it isn't very hard. You can tell what key something is in by just looking at the chord progression. If a song starts in Am and goes to E7, it's in Am. If it starts on a C and goes to G7, it's in C. If the chord is D instead, and it goes to A7, guess what key it is...

If the song starts on a Seventh chord(like G7), it is likely in the key of the chord that follows it, which, most of the time, would be a fourth above (or fifth below).

For that, you would have to learn what that G is the 5th of C, A is the 5th of D and such like.

Mostly (wet noodle time again) this is stuff you should have learned when you were learning to play.