How do you know if you have DVT?

Leg swelling and pain.

You’d know. It’s painful.
And the area around the pain would be warm/hot, and as @Rarebear mentions, likely swollen.

If you are reasonably healthy (not suffering from a clotting disorder, terribly obese or have other risk factors for DVT) and esp. if you are under 55, you are very unlikely to have DVT after a 12 hour flight.

Not sure if it’ll give you peace of mind—but I routinely do longer flights (ATL to Dubai was like 14.5 hrs..did a Cape Town, SA and Shanghai to MSP trip in the past year too..both had legs in excess of 12 hours) – and even without getting up to move around much/at all have never had DVT.

My uncle (who travels even more – and often to India from NYC) did not encounter DVT until he was in his early 60s..and the treatment for him to avoid it in the future is merely aspirin.