I just figured an easier way to prove, for anyone interested:
-Prove AC = BD (ie diagonals equal length)
-Prove m(AC) * m(BD) = -1 (ie perpindicular.)
Thus, Square.
...Well, if anyone's still wondering.
I can't be bothered to write anything out or find B, C or D, but I'm assuming you can do the following:
- Prove AB = BC = CD = AD
- Prove AB is perpendicular to BC or CD.
- Therefore, square. (equal sides, right angle.)