It’s pretty rare for the smiling and friendly person to be throwing subtle shade, that stuff usually comes from a more neutral initial presentation. In the event it does happen it can sometimes be difficult to detect, for sure, but I wouldn’t let those rare exceptions bother you too much.
Regarding detection, there’s almost always subtle tone and/or body posture cues that will mark it. They’re circumstantial though, and vary person to person as well. One of those things you learn as you get to know someone. None is surefire, either, the ambiguity is the whole point, after all.