Wow! Did you see that teenager run that stop sign? Teenage drivers are really pathetic.

I would personally say this is a hasty generalization. While teens tend to be worse drivers than those over 25 because of brain development, I don't think they are generally pathetic, especially not on purpose. 

So we have to fill in some gaps here. We are connecting the dots that this person thinks all/most teenagers are pathetic because of the one teenager who just drove through the stop sign. This might be the case, but doubtful in my opinion. I would think the person has a history of stories about bad teenager drivers along with a strong bias against them. Perhaps they are aware of the stats that "as a group" teenager drivers perform the worst (i.e., "pathetic") and that is what they are claiming. But this is just a guess—the way to know for sure is to ask them. Until we know, we refrain from saying they are reasoning poorly. There is no hard rule here though, because some implications are more obvious than others.

Would be the same if some said, "look at that black person robbing the liquor store. Black people are the worst."? 

Yes. Or anything similar that plays on stereotypes or is consistent with the data. Keep in mind the universal doesn't have to mean "every" person in that group, or not even "most." Universals can refer to common group characteristics that are simply more common in that group. For example, "Dentists are suicidal." The fact is a very small percent (like 6 percent) actually commit suicide, but as a group, this is the largest percentage. This is why ambiguous statements like this are so problematic—it can mean anywhere from a few percent to 100%.

How do we tell the difference?

[To Zakari]

Good question. Because some arguments can be implied , even a look can be considered an argument. However, I try to take the "give the other person the benefit of the doubt" approach and assume opinion over poor reasoning if not clearly stated, otherwise, ask for clarification. An argument has reasons, and it is in these reasons we can look for fallacies or poor reasoning. Without the reasons, we have nothing—we have a claim or opinion where we cannot justify the reasoning that isn't there.

See my comment to Electrical above. We don't know the reasoning, so we can't say that the conclusion doesn't follow from the reasons, because none were given—only a case of an observation.