Well, trolling is I think largely based on intent.

If one does not believe that they were trolling (i.e. they do not believe that they were doing what trolling is. This is a belief about what they were doing, not a belief about what the word "trolling" means), then that seems like fairly good evidence that they were not trolling.

I don't know what would be a better (and available) measure of one's intent than one's own perception of one's own intent. (Of course, one could imagine that there might be a better source, but in practice I think it is the best evidence we have).

This would only be as convincing to oneself I suppose, but I think that one's memory of one's intent is generally fairly good evidence to present to oneself to show that one was not trolling.

Sugar, fat and salt taste good. That means that humans and animals evolved to seek them out. If they were unequivocally bad for us, we would have evolved to find them disgusting.