I would be surprised if they didn't specifically test all participants in the trial for covid antibodies, symptoms or not. Does anybody know what validation was done?
Depends a lot on the question of whether the antibody test can distinguish the immune system's reaction to the virus from the immune system's reaction to the vaccine (or in this case: to the proteins built from mRNA blueprints in the vaccine). If they can't, and this is quite likely given that they all target the same spike protein, the outcome would hopefully be positive for all of the vaccinated.
In the Pfizer trial, all the subjects in the interim evaluation had symptomatic infection confirmed by PCR test. I'm not sure about Moderna, but it's likely to use the same or similar methods.