I assume vaccines produce slightly different antibodies to the various different variants of covid, and they can work out which is which, in a similar way as they tell the different between the various variants.
Oh, here you go, seems to be the difference between standard antibodies, and spike protein antibodies.
Covid-19 Antibody and Vaccine Immunity Test
monitormyhealth.org.uk
I find that a little confusing..
I took a covid 19 antibody test a few weeks ago.
And this was what the kit said..
It seems to say that spike proteins show in naturally produced antibodies and also in antibodies created in response to the vaccine.