* Adjective: It describes a noun. Example: "He is the supposed leader of the group."
* Past participle: Used as part of a verb phrase. Example: "They were supposed to be here by now."
* Participle adjective: Used as an adjective but formed from a verb. Example: "The supposed evidence was later found to be fake."
It's important to look at the context to determine the specific word class of "supposed" in a sentence.