Basically you got to know which direction the PTO end of engine is rotating. Most rotate CCW when view from PTO end. If you are viewing the flywheel side the same engine would be rotating CW from that view but is still considered rotating CCW as clutch is concerned.
Besides if you got the old PTO clutch it will have the OEM make and model number (part number) on it normally. You use that to find a replacement clutch; unless, you just buy one from the OEM of the mower.
I just did a discuss on another forum that got my thinking straighter on this rotation aspect.