Post History
Referring to history, as noted in other answers, is a good way. You don't have to depict the action to have it come up -- in conversation, when a character reads about something online, when a det...
Answer
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/32096 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Referring to history, as noted in other answers, is a good way. You don't have to _depict_ the action to have it come up -- in conversation, when a character reads about something online, when a detective turns up disturbing evidence, etc. You can also convey a lot by _other character's reactions_ to the character. If every woman in the room instinctively cringes and backs away when your harassing lech enters the room, that's a signal. If police, soldiers, or security guards reach for their weapons when your serial killer (who got off on a technicality) appears, that's signal. If your protagonists see someone being forcibly removed from a school with authorities shouting "we've _told_ you _over and over_ to stay away from our kids!", that's signal. None of this is giant-neon-sign-pointing-to-the-bad-guy levels of signal, but it's still signal, and it shouldn't take too much to get your point across. How much depends on how closely you're sticking to the tropes of your genre.