Yesterday my 8th graders had a written part of the ear training exam in school. Among other things, they had to write a one-voice and a two-voice dictation, each 8 measures long. Both were in F major.
At the beginning I played treble A and the tonic chord of F major key. The students were supposed to discover the key from this A by themselves and write Bb next to the clef.
All of my students got the key right but one. He wrote G major with F# next to the clef.
So G major or F major? What's the difference?
Here's the thing: In F major tonic chord the note A is the middle note of the chord (FAC) whereas in G major tonic chord A is a whole tone apart from either G or B which are the chordal notes in this GBD chord.
Does it make sense?
Treble A: GBD vs FAC?
Try it on the piano and listen to the difference.
If you can't get it right, play A and then ANY major or minor tonic chord and practice for a while.
You will get better with time. Doesn't matter if you get it right on the first try. Doesn't matter if you have a perfect pitch or not.
Perfect pitch is a result of our memory: the more were are exposed to this treble A, the more likely it is you can remember how treble A sounds.