Here's a few suggestions, some sillier than others, in no particular order:
Much depends on the audience one is signalling to.
Join organizations like Mensa
To stupid or average people, this is a signal of intelligence. To other intelligent people, my impression is that Mensa membership mostly distinguishes the subset of "intelligent and pompous about it" from the larger set of "intelligent people".
Associate yourself with games and activities that are usually clustered with intelligence, e.g. chess, Go, etc.
Again this works as a signal to people who are at a remove from these activities, because the aver...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
(I plan to make these threads from now on. Downvote if you disapprove. If I miss one, feel free to do it yourself.)