Reach: adds a number to the amount of "range" you get on an entity while attacking.
How to detect: if it's blatant and you can't hit the other user whil he's above 3 blocks away from you it's probably reach. Try downloading: "Tenebrous' Reach Circle mod" this shows from how far the other player can hit you, adn from how far you can hit them. [This is only visible and doesn't give any advantage, this just let's you know or you're fighting a cheater.
