How are you supposed to know when a shower cartridge is broken?
When the shower’s water flow doesn’t operate like you want it to.
A shower cartridge sits in the valve behind the wall. Think of it as the maestro during an orchestra. It controls the flow of both hot and cold to give you the temperature you are looking for.
Many people think that if the hot and cold water is reversed in the shower, that either the plumbing is backwards or that the cartridge is bad. It usually is the result of the cartridge either being installed incorrectly, or that the stem just needs to be rotated 180-degrees.
So to answer the question more specifically, the cartridge is broken if you turn the water off and it continues to run or drip. That is a broad brush stroke, so let me say this: your cartridge may also be considered broken if when you turn it to “hot” and the water pressure slows down to a drip. That is very common with a Moen Positemp cartridge. You can replace the cartridge or just clean it up. You can pull it apart and clean out all the deposits in it. Generally, if you can shake the positemp cartridge and it rattles, you are good to reinstall it.