I seen you were going to go look for pics of Fred LOL, I tried to find close ups I had at work to get their eyes but this was all I could come up with. Fred (JRT) has very almond shape eyes where Alice (Rat) has very round eyes. Their bone structure in their head seems different as well.  2 Jacks and 1 Rat in the last pic.

In the 3rd pic Fred & Alice look very similar to me (like Rats) and the one on the right is an obivious JRT.  I would have never known that Fred is a JRT.

I think it's really the eyes that are most different. Fred and Alice have the same coloring but if you were to see them in person they don't look so much alike!