* Adjective: This is the most common usage, describing someone who is good at noticing things. Example: "She is an observant child, always noticing the little details."
* Verb (past participle): This is less common but can be used to describe something that has been observed. Example: "The scientist's data were observant of the changes in the weather."
It's important to consider the context of the sentence to determine the correct part of speech.