Depends. Usually by their fur, most times there are some differences, might not work with a whole litter of the same colour. Sometimes by behaviour.
My 3 black rabbits have different bodys, and head shapes. The two agouti kits, 10 weeks now, are mory tricky, but one has a little more bushy fur at the leading edge of her ears, also that one is more skittish. The other two kits, broken agouti, look almost the same but one has a white dot on the tip of the nose.
If I need to know apart very similiar rabbits I mark them, short term with permanent marker on the inside of a ear, long term by cutting a small spot of fur bak, about to half lenght. visible with colours like agouti, I doubt that would work as well with white rabbits.
Ear tatoos would be a permanent option, like it's done with show rabbits.
