the secret ballot is sacrosanct in the US, yes. nevertheless, you can tell that a voter is a registered D, I, or R. you can tell if they turned in a ballot in

the secret ballot is sacrosanct in the US, yes. nevertheless, you can tell that a voter is a registered D, I, or R. you can tell if they turned in a ballot in a given election. in jurisdictions where registered voters cannot vote for candidates of the other party (many), then yes, you can then reasonably infer their vote, with a margin of error equal to rates of write-ins for that party in that election.