As others have said, with the bike "level" which is not on the centerstand. That said, it is really not that critical- I fill mine on the centerstand with the front wheel on the ground and try to stay toward the lower mark. When you push the rear end of the bike down, the oil in the sight glass will increase but not by that much. If you fill it until the level is in-between the marks on the sight glass, even with the front wheel on the ground, it won't be appreciably overfull when level.
Besides, what happens to the oil level when climbing hills? How about going down hills? See- it really is not all that critical.
Brian
So what is the proper way to check oil through the sight glass, is it up on the center stand in the normal resting position (front wheel on ground, back wheel up) or is it when the back wheel is on the ground (by pushing down on the back of the bike to lift up the front wheel)? I know it's a basic question, but I just want to make sure my oil level is correct.