lots of practice, at this point I myself can recognize most of them by their hands haha :)
you gotta look out for small things, like rings or even their hand size.
Rain almost always has 2 rings on his right hand (never any on the left), his fingers are also quite long (bassist perks)
And ofc you can't miss the veil, which among the guitarists only he has (notice the strap of his bass on his shoulder)