They are told about the vaccine at the time they get it. That is part of it. Most people will be asked if they have had a severe allergy. They will be asked if they've ever reacted to an influenza vaccine or to thimerosal.
Now, the unadjuvanted vaccine we've ordered from CSL also has antibiotics in it: neomycin and garamycin. They are not in the other vaccines but they are in this one. So they would also be asked about an allergy to those antibiotics.