Very often a service dial is from a different era than the watch and looks completely different than the original dial, e.g. a luminova dial on a watch that originally had a tritium dial, or a WGS gloss dial on a watch that originally had a matte dial with painted lume plots. You can easily google many photos of Rolex watches with service dials and compare them with known examples of original dials. Just takes a little research. But there can be many different service dials for the same references, because difference service dials were used in different eras. A service dial basically kills the collectible value of a vintage watch, but is perfectly authentic and a correct replacement according to the brand.
Instead of asking such a broad question, I would suggest narrowing your focus to a particular model / reference to develop your expertise. If you are serious about buying a particular sport Rolex, you need to narrow down your search and start learning specifics.