The Right Hand Engine will be the critical engine assuming that they rotate counter-clockwise when viewed from behind. (Left engine if clockwise)
Why? The down going blade creates more thrust than the up coming blade (Needs a picture to explain why). The Left Hand Engines down going blade is furthest away from the aircraft with the right hand engines down going blade being relatively close to the fusalage. That means that if an engine is failed the left hand engine causes more of a yawing moment than the right hand engine because the highest amount of thrust is out on the wing and a long way from the fusalage. The greatest amount of thrust on the Right Hand engine is close to the fusalage and the center line of the plane. All this means that losing the right engine is worse than losing the left engine and therefore the right engine is the critical one.
Most engines rotate clockwise so left engine is usually the critical engine.
Much Easier to explain with pictures, perhaps run a search.
Try this link.
http://www.fergworld.com/cfi/pdf/Mul..._Reference.doc