If you want to know whether you can backstab with a particular weapon, just equip it and try to backstab. You'll get a message saying "cannot backstab with equipped weapon" or words to that efect.
I found this out over the weekend with Imoen, with staff of magi, was invisible and whacked a monster.

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"Butt-kicking for goodness!" - Minsc
"Cities always teem with evil and decay. Let's give it a good shake and SEE WHAT FALLS OUT!!" - Minsc
http://www.dabros.net/images/tcampbell/hadb6copy.jpg

I would have, but I wanted to know, for purposes of character creation, before I got to the point in the game where my character actually had backstab abilities, so I wouldn't have to start over again should I have screwed up and assigned my character contradictory skills in backstabbing and a weapon with which you can't backstab. This is my first time through BG2.

In case anyone's wondering, I did go with five stars in katana and one star in two-handed style. I seem to be enjoying the character thus far. Thanks for the advice, everyone!