Yeah, visual, to a point. Just sight down alongside the head tube to the bottom of the fork. If the fork is bent it will be at or near the crown, and the upper portion of the fork arms will be not in a straight line with the head tube. However the OP asked about telling the difference between a bent fork and one that was made that way, slightly off. I don't see that there is any difference, certainly not one you could see or measure unless it was so bad that the fork arms had rippled metal. The fork arm/HT alignment and the wheelbase and the distance between tire and DT will be whatever they are regardless of how they got that way. If it seem like so little that "it might have been made that way" then the bike should be just fine.