How to know the direction of the force measured by a the Contact_Sensor?

and how do you know where the object is with respect to the zero frame?

Hello Wayne. The Contact sensor is built in a way to model pressure sensors as they are in the real world. They only give the scalar value.
Just like in real world, you’d need more than one pressure sensor and do an interpolation to find the force direction.

With that said, you can totally get the direction if you use the Raw data, as that is a list of all vector forces in the rigid body, with their global position. To get the relative position to the rigid body, simply do force_pos - body_pos.
Check the Sensor API to get the Raw data

to get the rigid body pose, you can use the omni.usd.get_world_transform_matrix