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.
Yeah, that second line is the sneak attack damage.
The formula for how many sneak attack dice a rogue has is ((Level - 1) / 2) + 1, always rounding down.
So, on levels 1 and 2 you have one sneak attack die, on levels 3 and 4 you have two sneak attack dice and so on...
The problem here, that may be the reason for the confusion, is that the game, instead of rolling the appropriate number of dice, instead rolls only one d6 and then multiplies for the amount of dice (as if they all had landed on the same value).
That's what the second line reads. 4 (1d6 Piercing) means a single roll of a d6, resulting in a 4. then multiplied by the formula to know how many dice.
