I've got to agree with Stealthee on this. Not only is turning the engine backwards dangerous but its pointless as well. When you turn the crank clockwise it pulls the belt down from the fixed idler pulley and rear exhaust cam allowing the tensioner to maintain it's normal position. When you go counter-clockwise you pull the belt against the tensioner pulley which then pushes in against the tensioner, collapsing it. Even if you did find the belt loose after rotating backwards all that would mean is that you pushed in the tensioner. The only real way to test this is to rotate the engine around a couple of times (CLOCKWISE!), reset to TDC, and measure the protrusion of the tensioner.