Well, if you mean telling the different Breeds apart, do a google search of the types that should be helpful. If you mean telling the individual chickens apart...
I have three groups of chickens, 2 each of 4 different breeds. The older ones have slight differences in coloration. Of my barred rocks, one is darker than the other, black and white while one is more of a grey and white. However the best way I tell them apart is their personalities. One of my barred rocks is tamer, calmer, more inquisitive. The other is more aggressive and less likely to be friendly. The same for my black australorps, one is much friendlier, much more inquisitive. The other is quieter, more likely to settle in one spot and lay around watching the others, rather than joining in in their dustbaths, etc. These 4 are 7 weeks old and are definitely distinguishable by their personalities.
I have two welsummers that are now 4 weeks old. They are pretty much indisguinshable by looks at this age, and are just now starting to show personality types (1 wants to hang out with the big girls, 1 wants to hang with the 2 remaining baby chicks I have). I suspect that as they age, it will be personalities that I rely on to tell them apart, more than looks.