On any given day, the number of people attending even large protests in a region also probably pales in comparison to the number of people who are regularly exposed to others through their public-facing jobs or daily routines, especially as the country begins to reopen.
That said, if any outdoor activity could noticeably raise the risk of catching covid-19, it’s probably a large protest where people are bunched together and shouting loudly, with or without masks.
On the other hand, the ramping up of contact tracing, where investigators try to identify any potential sources of exposure through detailed interviews with confirmed cases, may directly ask people to reveal their protest history.
Because of the inherent risk that comes with spending time around large groups of people, yes, it’s in everyone’s best interest to get tested in a week’s time if you’ve recently attended a protest.
Just remember, you can contract the coronavirus and spread it to others without feeling sick, so having no symptoms a week after your last protest isn’t a guarantee that you’re covid-19 free.