As far as I know, there isn't a way to tell directly. However, we can approximate from the difficulty using the following relation:
At the current difficulty ~ 0.6482, that means we can guess 4640000 hashes/sec, i.e. 4.64 MHash/s.
EDIT: Hold on, that's clearly wrong. Let me figure out what the correct equation is.
TY
Well, I can't find the information anywhere. I'd have to dig through the YACoin code, I suspect.
That relation is valid for BTC and LTC and their derivatives (Bytecoin, FTC etc), however Novacoin derivatives like YACoin have some weirdness with their difficulty adjustment (like varying the block reward), so I'm guessing that's why the difficulty/hashrate computation is inaccurate.
Sorry I wasn't more help. Maybe someone more knowledgeable has the answer.