The player could describe what they saw, but nobody except the GM knows if it's true.

It's equally plausible that the paladin failed the check and saw a monster when there was only a dog, or that they passed the check and saw a monster because there was in fact a monster. Their argument to the party would be the same in either case: "that's no dog, it's a ~~space station~~ monster".

The party then must question who saw the correct thing. Did the paladin actually see something everyone else missed? Or are they just seeing things? My point was that the players should not immediately be able to discern the truth. I find that this kind of uncertainty breeds intrigue!