I had delta in late October and omicron in early December. I was PCR positive for delta, although I had a mild case. I wasn't allowed to take a PCR test for the December infection, because it was too soon after my delta test and my symptoms were very mild. I did test positive multiple times on LFT, my symptoms matched the 'ultramild' omicron (scratchy throat, slight cough, lower back pain). The day I tested positive for omicron was 4 days after my friends had gathered in a pub, of the 30 of us in attendance 27 tested positive on the same day. The group chat was wild. Some of those that got PCR's were informed they had omicron.

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.