I haven't come across unimodular matrices before, but according to the definition on wikipedia it seems fairly straightforward to implement a naive algorithm to decide it.
Also, have you considered trying IRC for these sorts of short questions? The #haskell IRC channel on freenode is helpful and seems better suited to asking questions like this, if you just want quick answers. There's a shorter lag than a mailing list, and the IRC medium seems closer to the kinds of things I've seen you ask.
From observing the mailing list for quite a while, it seems that if you want a decent conversation on here, you should ask more "fleshed-out" questions with an explanation of some background to the problem, and more details on what you want and why.