How to measure force that applied on grab bar
My first knee jerk reaction is to use load cells at the mount points.  If you want to measure the direction of the force (down or in/out from the wall), which you probably do, then you probably want two load cells at each mount.  One above the other should give you enough information, although this won't include force lateral to the wall.

I don't see how a clamp on sensor could measure the information you seem to want.