A6 or relative minor

If the notes are the same for an A6 and it's relative minor, how do you know which one you are useing? Isn't the A6 an inversion of the F#minor7

It depends on how you look at it.

It's also relative to DMaj7

  A6    F#m/C#    DMaj7

e--2------2------2---

B--2------2------2---

G--2------2------2---

D--4------4------4---

A--0------4------5---

E

The bass note most time gives the key but that is not always the case.

It's your choice what the root is. Any chord is actually several chords at once. Each note is root to a different chord.

And no @MoshZilla1016, the bass note does NOT give the key. It gives the key of a possible chord which may be the chord you had in mind. Think beyond that. break chords up in intervals and their inversions. Understanding chords becomes much easier and you'll have much more freedom in playing. It works in melodies too.

I gave 3 examples with the middle one being F#m/C# with the statement..

so you said...

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...*facepalm*...