Key Change With Capo

[Tele Master]

When you put a capo on, how do you know what key you are now in?Say the song is in key of Amaj. If you put a capo onto the second fret, what key are you now in?

[Slow Diver]

The capo does not change the key. It rather changes the open string chords. Nothing more than a permanent barre.

[Tele Master]

So, for example, would a Dmaj chord become an Emaj chord if the capo was put on the 2nd fret?

[Slow Diver]

Absolutely
Having a capo on 2nd position will allow you to play in open positions chors like F#, B, E, A and etc
Pretty logical
For example you might want to play Knockin on heaven's door in A, and without using a capo you will most probably have big trouble playing it without altering the original shape