would a resistance test tell me whether or not an RPM sensor is bad or failing?

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Probably not. Hall effect sensors rarely fail as such . . . they or the magnet that triggers them could fall off of their mounting or the electronic circuit could fail.

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Hall sensors do not rely on resistance changes, so there is no appropriate test to perform.

You need to determine if the hall sensor is generating current. It is quite possible that the sensor module also contains an amplifier, which could also break.

A hall sensor is quite electrically inert, as it is a simple metal foil with wires attached. However, these wires can be broken (via thermal shock, most likely).