For the Cessna the datum point is somewhere near the back of the bonnet
... which may vary if the plane is further from the car park.
Seriously.... the datum for any plane is specifically stated in the POH and the weight and balance forms, and very easy for the pilot to find. For single Cessnas, it is usually one face of a stated portion of the firewall, which can be accessed through the cowling. Other types use other datums, some forward of the plane. I have never encounter a datum behind the C of G of an airplane. The moments can be calculated relative to the datum as stated. Understand that weight and balance moments, which the pilot must calculate, will differ from the aerodynamic moments. The pilot should be aware of the aerodynamic moments, which affect how the plane reacts in a stall, while very certainly respecting the weight and balance moments, as those assure compliance with the aerodynamic moments.