Unlike his family, Remus knew people made mistakes and had to learn from them. No need to be cruel.

"It depends on the quality of the illusion. I've worked hard for that illusion - I had to be able to hide my wand from the other werewolves when I went into hiding with them." He gave a small shrug. "However, outside of that, there's a very easy way to test for such a thing."

He held up his hand. "If you cast a simple spell, something that should hit them - watch their bodies. If they have an illusion upon them, the part of them that is illusioned will glow alone if they have no protection spells on, and will not glow if they do."