Running rich would cause the motor to bog and have a strong hesitation when you hit the throttle. Couple of things to look for:
1. Excessive smoke out of the exhaust. Note that you will always have some smoke, but you shouldn’t be creating a cloud.
2. Oil/fuel dripping out of the end of the silencer. Oil/fuel leaking between the manifold of the engine and the pipe (causing a messy block!). Oil/fuel leaking between the silencer and pipe.
3. Oily gummed up plug. The plug will go black over a time, but it shouldn’t have a build up of goo.
Running lean or correct, you’d not see any of the above. But running lean you’d see the plug burning itself up/whitish, you’d see pitting in the piston top near the edges, you feel like the engine was zingy (basically it’d feel ‘too’ responsive).
In that photo you can see pits all around the edges of the piston. BUT, note that this same phenomenom can come from poor cooling (which is the same as running too lean, the oil cools the system).