Looked like water. I think the only way to get a good idea is to have an antibody test in a few weeks.
Does accessing antibody test results break the double blind? Are you sure you didn’t consent to not doing that?
Short answer is yes, it mostly does, but they’ll test me anyway. I didn’t have to agree to anything that could possibly prevent me from receiving the best healthcare I would otherwise get. I’m pretty sure/hope such a condition would not have been allowed by any IRB.
I spoke with a pathologist in my family and together we estimated that a positive blood antibody test ~2 weeks after the 2nd vaccine dose would have a likelihood ratio on the order of 50:1 or more in favor of having had a response to the vaccine. (This is specific to me—my lifestyle protects me very well from possible infection (I have close contact with very few people, most of whom have received a vaccine themselves or have recovered from covid a couple of months ago); I would reduce this estimate for an average person). So if we find antibodies in my blood about a month from now I would be quite comfortable concluding I received the vaccine (recall the prior odds are 2:1 in favor, updating based on the test makes it 100:1; a near certainty). Likewise a negative blood antibody test would strongly I did not receive the vaccine.
You are way better than average at avoiding infection then. CDC estimated that from February to December that there were a total of 83.1 million infections in US, over 20% of population.