And here we have the problem... they're so controlled here in the US that allergists can't get the shots for monitoring.
Typically, if I wanted a vaccine (other than the flu shot since I know I can safely get that) I would go to my allergist. He would skin test me, and then if I had no reaction, they would administer the vaccine and monitor.
For this shot, you're supposed to get it in a hospital. We'll, most hospital clinics closed down in May. I was on a wait-list to get in...and they shut down.
So only one hospital (Hopkins) had them, but my allergist said absolutely do not get J&J or Pfizer, only get Moderna. Hopkins only had Pfizer and J&J unless I wanted to go to their campus that wasn't in a particularly safe area in Baltimore.
And then I asked my doctor's office about doing it (my primary care doctor died in June...I've begged them to just reassign me a primary care doctor and they've refused). They wouldn't do it either because of the allergic reaction history.
So I finally went to Walgreens and fudged my medical history so they'd give it to me, and I loaded up on the Benadryl (because to your original point, how would I know if I'd react? Didn't know I'd react to that other vaccine. Didn't know the dyes in my thyroid vaccine would make me break out in hives. Didn't know I was allergic to papaya until I had a papaya green tea at Panera. It was a very distinct possibility). Then I went home and waited for hours near my epi pen. It was needlessly terrifying and I don't recommend it for anyone. If I had it to do all over again, I probably would have done some more insisting with my doctor's office that we find a way to get it done at their associated hospital.
And I did get tired of all of the judgement about why I hadn't had it done or what I was doing walking around unvaccinated and still doing stuff, which is why I will fight tooth and nail to defend people who say they have a medical exemption, because we're the ones who want the vaccine and not the problem.