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
