For me, the sneak-attack damage is shown as a different line in the combat log. It does not say it's from sneak-attack, but I attacked once by clicking the sneak-attack skill first, and I got 2 lines for damage dealt. I had 1 dagger equipped.
I have noticed that the sneak attack damage is in a different line too (in the off chance that it somehow actually works) but I don't get the damage about 90% of the time that I meet the criteria for sneak attack.