Depending on how bad it is something in the range of:

 - Fix it yourself (if you can just about understand it - but only if you're sure you're not altering the meaning)

 - Leave a comment telling them you don't have a clue what they're talking about and that they should edit their question to explain a bit more.

   When I say comment, I *mean* comment - not answer. You'll need 50 rep before you can do this. 

 - If it's so bad that there's no rescuing it (and/or the OP seems to have no interest in fixing it) mark it for closure as Not A Real Question. If you don't have that ability yet the lazier moderators (ME!) would appreciate it if you left a comment for the OP to fix the post and gave them some time before flagging it for our attention.

   If it's already old (and the OP has ignored any prompts) when you get to it, flag it up.