[Tropo-Bob (Bob)]

I have been observing Saturn's moons with a variety of small refractors. I can see Rhea (2nd brightest moon) with an 80mm scope, although 100mm is better. It is interesting to follow each evening, as it moves so rapidly around the planet.

However, even with a 100mm, Dione and Tethys are rather difficult and I will need to use a bigger scope to follow these more seriously.

From what I have seen though, these both look rather alike. Can they be told apart easily? I know each of Jupiter's moons by sight and would like to obtain the same skill with Saturn's Moons. If anybody can do this, please advise if the slight difference in brightness is the clue, or simply the moons distance from Saturn.