Here are a couple handy tricks when trying to figure out diminished chords--

First:when you see a Diminished chord, such as an Adim, you can often play the Seventh chord that falls a major third below(two whole steps) which would be F7--this is because, if you take the F out of an F7, it is an Adim--

Second,you can often just slide whatever chord fingering you are playing down a half-step and pluck the third and fifth notes in the chord--

Third, you can take your A fingering, and just move the third and fifth steps down a half--take this open A: 0-0-3-3-3-0 (each number is the fret you play, 0 is open, X is unplayed, Low E is the first, then A, etc) then change it to X-0-2-3-2-X.

Your A chord is A-C#-E, and you are just moving the C# down to C and the E down to D#--