yep haha, i think this is year 11 stuff right?
all you need to do is find the gradients of AB, AC, AD, BD
if ABCD indeed is a square mAB=mCD and mBC=mAD
where mABxmBC=-1 (since its a right angle - perpendicular)
and then you can use distance formula for AB, BC, CD, AD - should all be the same...