"just look at his ap..."
-_- "his stacks don't have a special interaction with his skills like nasus' stacks do" -______________-
yeah you're right, it just increases every single one of his damaging spells but "it has no interaction". The only thing that nasus get is more damage on his q.
And yeah you can look at his ap but as far as I know, veigar also builds ap and not everyone wants to make the maths (even though they're simple) because you don't know how he runed himself etc.
Seeing how much he has farmed his Q is a good indication of knowing if you managed to stop him from farming it or if you failed completely.