There is also the possibility that Crouch knew Moody a bit as his father's acquaintance during the war. Crouch Sr was Head Auror/Head of MLE during the war, and Moody earned his fame during such. Its not a complete stretch to assume he might have been over the Crouch Sr's house a few times.

Also, maybe its possible to forcibly steal memories too.

 

If Crouch had some talent at legilimency (or had Voldemort do it for him) it would go a long way to explaining his acting skills.

 

The false image of Moody we saw in GoF was a caricature, rather than a normal person. He had extreme, regular, and recognisable quirks that would easily distract people from any discrepancies with the personality behind them.

I also imagine that Moody was not particularly sociable, so he hardly would have been expected to spend any time with the other staff members. The students would have no possible frame of reference for him being an imposter.

Crouch Senior is shown to be obsessive, and I could believe that such a nature was inherited by the insane Crouch Junior. It's the kind of personality that lends itself well to such a task. He might well have revelled in it, taking a similar thrill out of walking around Hogwarts undetected as he would out of torturing Neville's parents. He had power over everyone in that school for that year - they were completely vulnerable to him, and never knew it. A certain brand of psychopath would get a real kick out of that.

Crouch Junior is one of my favourite Death Eaters. He's insidious, cunning, and succesful. Far more interesting than a snooty aristocrat, vicious thug, or cackling madwoman.