The behavior, mostly.
At a certain size, if they allow each other in a near vicinity, they're likely a pair.
The other good indicator is that every other similar Channa is either dead, or hiding for dear life.
As for buying a pair at a LFS, that's very unlikely, unless your LFS sells grown ups, and in that case, it's the two big ones alone in a tank to themselves. But since LFS don't have the proper setups for channas, that's quite hard to actually see.
@OP: Your pictures are as amazing as your channas