Personnally, a part from very obvious case, I don't really rely on this method because it's a bit misleading for me... So I'd like more precision in how we can distinguish between the two....
A general rule is that P-lead's eyes look as if they're irrelevant to the rest of the face. My eyes almost always look the same. Even when I'm angry they stay "Ni-like" (imagine someone with "bored/hypnotic-looking" eyes screaming at you.. it looks quite "off"). It's almost as if someone cut them out and paste them on my face.