Being confused often by people and not being able to apply yourself to simple intellectual tasks would be good hints but they could also indicate other underlying disorders and atypicalities different from intellect.

This is further exasperated by the fact that most people specialize in one or more brainy tasks such as mechanical visualization, language comprehension (spoken and/or written), rhythm, mathematics, etc.

There are also behavioral differences that are judged as more or less smart depending on the person. For example, is selfish self serving actions smart or stupid? Do we prioritize long term satisfaction or short term? Does a smart person care about betraying trust of others?

TLDR: Idk man, its real hard to tell, dude