DD was breech from about 28 weeks, and was born as a breech baby at home with IMs! - even if your LO is breech and doesn't turn you don't have to have a CS as long as you can find MWs with good breech birth skills.
Anyway, that aside, I was told that one way you can try to distinguish a breech baby is if you put one hand on either side of the head/bottom (at the top of your bump) and push gently with alternate hands - if you've a head down baby then the bum and the entire back will move from side to side, but if its breech the motion will stop at the neck (apparently its "ballotable" - like the basket of a hot-air balloon).
In theory you may be able to tell from where you hear movements - spinning babies has some info on how to map your baby, although I could never work out what was where with DD.
Moxibustion is definitely worth starting earlier if you can - I think the studies that showed some success suggested starting treatment at 32-34 weeks.
Another theory is that an asymetry in the woman's spine and/or pelvis causes a torsion in the uterus which makes it harder for the baby to turn. There's a chiropractic treatment called the Webster technique that aims to remove the asymentry and release the tension, freeing up more space.
Hypnotherapy has been suggested, as has inversions (which you're already doing - you could try doing handstands in the swimmingpool as well), trying to persuade the baby to turn away from something unpleasant (sitting in a warm bath with an icepack on top of your bump or turning a cold shower on the top of your bump), and trying to persuade the baby to turn towards something nice (shine a torch, play music or get your DP to talk where you want the head). It can all be entertaining even if it doesn't work
ECVs aren't normally performed before 37 weeks. Yes, the baby will turn more easily if they're small - but they will also turn back more easily. There's also a small risk that the procedure can either trigger labour or cause distress to your LO, which could necessitate an immediate CS, hence they aren't generally performed before a baby is considered term.
Lastly, though, your LO is more than likely to turn of their own volition before 36 weeks - especially as its not your first. DD was my first and a big baby (9lb 12 when she finally made her appearance 10 days late). From my experience, because most babies DO turn before 36 weeks and because the NHS won't do anything active for a breech baby before that point (because most of them will turn by themselves, and moxibustion isn't often available - worth asking though) they'd be unlikely to give you a scan at this point in time as the knowledge won't tell you what your options will be and won't enable you to do anything about them.
If you want to do some reading on choices just in case you LO doesn't turn, I found "Breech Birth" by Benna Waites and "Breech Birth: What are my options" by Jane Evans to be very approachable, balanced and informative.
Sorry for the essay and fingers crossed your LO gets themselves nicely head down before your next appointment