"Probably", I'd say. On rare occasions it may be used as an adverb meaning "for sure" ("I know this for sure"), but I perceive that as an old-fashioned use.

You are most likely to only ever encounter the word used as a parenthesis, when it always means "probably/ most likely". It is easy to distinguish between the two. If the word doesn't seem to have any grammatical connection to a verb in the sentence, then it is parenthesis, and then it is "probably". Which is, like, 99% of all contemporary use.