sort of related: it drives me crazy when people use a question mark to show doubt, (sometimes sarcastically), instead of to ask a question. i've never seen (?) before, but i see "i think mark said he was done?", a lot.
even though it drives me crazy, it shows that special punctuation is unnecessary. people can just keep doing that awful thing, and there's no real ambiguity.
Many people raise the pitch of their voice as if asking a question when saying something doubtfully, so the question mark reads similarly to how one would say it out loud.
If you have access to formatting, a similar way of achieving the same thing would be to italicise "think"
(!) - Drawing extra attention, maybe belittling: "He wore sandals(!)"