I'm not sure when this would come in useful, but you can check to see if you're hidden by trying to list items at the BG auction house. If you are hidden, the auctioneer will start peering around. If you aren't hidden, it will list the items. After this I tried it out at the inn too. I guess this happens for most interractive mobs, but kind of useful to know.
< 257h/257H 119v/120V > list tail
Auction list for tail only

item#)    item id                                item description    bid bidinc
       top bidder time  status seller
====== ========== =============================================== ====== ======
================= ==== ======= ======
< 257h/257H 120v/120V > hide
You attempt to hide yourself.
< 257h/257H 120v/120V >
Autosaving...
< 257h/257H 120v/120V >
< 257h/257H 120v/120V >
Nitupopple says 'where the hell is azuth when u need him'
< 257h/257H 120v/120V >
Autosaving...
< 257h/257H 120v/120V > auction list tail
An auctioneer peers around intently, looking for something.

The Library

[Yarash's image]

[Yarash's image]