Well, it is easy to detect the real lancer with legion coz of his first spell. Overwhelming odds dealing bonus dmg to illuisions and summoned units equal to 25% of their current HP, before the normal dmg applies. That means he can make much more dmg on the illuisions in a 330 aoe, so the huge difference in the HP loss after his spell, will clearly show him which one is the real one.
Can you provide some exact situation, with replay times when you think he used maphack?