Set your scope in a v block and look through it at a mark on the wall. As you turn the scope the crosshair will move off the mark(if the erector tube is not centered). Adjust the evelation and windage until the crosshair stay centered on the mark.
Another way is to place the scopes bell against a mirror and while holding your eye centered of the image, crank the adjustments until the crosshair reflection aligns with the crosshair. You can either do just the horiz as requested or both to find the optical center adjustment of your scope.