I can often tell chickens apart by appearance, especially after they finish growing up. Even when they are the same breed, there might be one hen with skinnier points on her comb, another with wider points, another with a comb that slightly flops, feather color might be lighter or darker, patterns in the feathers might be a bit different, and so forth.
For telling apart two chickens that look the same, I like to cut some of the tail feathers shorter on one of the two. (The feathers need to be cut again every time the chicken molts, so it happens frequently as the chicks grow. Adults only need re-trimming once a year.)
I think a chicken looks a bit ugly with the feathers on the tail cut short, but I like that I do not have to change legbands as the chicken grows. If I forget to trim the feathers, I lose the identifier, but the chicken is not injured. If I forget to change legbands, the too-tight band can badly injure the chicken's leg.