We need to distinguish between judging actions and judging people. To take a very obvious example, if you see someone killing another or robbing a bank, you must recognize the evil of the action. What you may not do is make assumptions about the dispositions of the person doing the bad act - you don’t know if he truly understands that it’s wrong, you don’t know if he’s acting with complete freedom, in summation, you don’t know if he’s really responsible for his action - you cannot judge the inside.
Now, as to YOUR sinfulness in doing this…if you just have a passing thought that the punky looking guy might be up to no good, that doesn’t really cause him any harm, and would probably be, at worst, a venial sin. If, on the other hand, you base an important decision about him upon your impression of his appearance and your judgement that he’s probably dishonest or bad, and he suffers as a result, that could be a more serious sin on your part.
If you see another person commit an evil act, you must recognize the evil inherent in the act, you can be concerned for his salvation, and you have a duty to pray for his salvation. That’s not the same as saying, “He’s going to hell.” You cannot know that, but you can fear it and pray for him.
Betsy