The easiest way to identify a rex-type rabbit is by touch. The rex mutation changes how rabbit guard hairs are formed, making them either shorter than the normal undercoat or absent entirely. The result is a fur-type that feels much softer and denser. You can see this described as "plush" or "velvety." In fact, the first name of this breed was Castorrex, as beaver fur is known for its density.
The fur is also supposed to stand up straight. A good cue for this is that it feels a little like touching your hair after you've buzzed it close to your scalp: not springy, exactly, but the tension of the hair itself is not overcome by the weight of the strand.
Additionally, if you know the breed of your rabbit, the National Rex Rabbit Club recognizes 16 varieties of rexed rabbits. Of course, rex-type fur can appear in any rabbit but this is an additional way to cross-check your identification.
If your mystery rabbit and your definitely-not-rex-type feel the same, my guess would be that you have two rabbits who are not rex-type.