Most PCR tests will discriminate between (B.1) Omicron and things which are not B.1 Omicron, so it's feasible to know this. I mean, strictly speaking all you'd know is that you had B.1 Omicron plus something else, but if you had the non-B.1 Omicron one in October, then in most parts of the world you'd be pretty safe in assuming that was Delta.