On Concorde, you needed clear conditions, and really be above 55,000 feet.
So best seen on a BGI
sector, usually clear when you reached top of descent at 60,000ft, as the longer BGI
sectors usually reached this altitude, as opposed to 58,000ft maximum on a JFK
Certainly, of my 7 flights, the best view was on mine and G-BOAE's final flight, last November to BGI
To get the best discernible view you needed to look through 2 to 3 windows, positioning your head a few inches from the nearest window and looking through the immediate ones ahead of you as well.
Easier done on a nearly empty air test as I did on OAE last August, positioning yourself you are not in anyone's way or infringing on their personal space is do able if there are only about a dozen people in the whole cabin.
Even given the differing interpretations of what the curve of the Earth really is, if it can be seen on the ground or only in orbit or sub orbit, the sight from Concorde was sufficiently different compared to a normal view from an airliner, given good conditions you could make out a curving horizon in a way not possible from lower levels.
I saw it clearly on five flights, not as definable on one, difficult to make out on one.