I don't know what you mean " clear reading ohms ". If you probe the neutral bar and the panel metal where
it is clear of paint you should close to it. If you get open or high ohms the the box is not bonded.
Normally the green screw is used to bond the neutral bus bar (terminal strip) to the panel but we cannot
rule out an incorrect neutral to ground interconnection (bonding) somewhere else in the system or a ground
fault elsewhere in the system. If you undo the green bonding screw in the neutral bus and then get near
infinite resistance between neutral bus and panel frame, then re--insert the green screw and get zero
resistance between neutral bus and panel frame then the screw is accomplishing its job. Any sentences
beginning with "otherwise" you put after this are not proven true or false by the preceding ohmmeter tests
e.g. "the converse is not necessarily true."
In some cases a fat wire or a metal bar (bonding jumper) is connected between the neutral bus and a
ground bus in the panel and this accomplishes the same purpose as the green screw. If it's a fat wire
it should be of a size at least that of the largest other ground wire (equipment grounding conductor)
attached to that ground bus.
If neutrals are attached to more than one bus then that procedure must be approved for that model of
panel and those busses must be interconnected (bonded) with an approved bonding jumper for that panel,
not necessarily "just a fat wire." In the picture above, multiple busses attached to the same metal subframe
are properly bonded (to each other). The green screw adds the bond to the panel when needed.