Broken needles cause holes, ladders or broken stitches in your knitting. The easiest to spot, is a hook that has broken from the needle. A damaged latch can be a troublemaker too. Sometimes they get crooked and no longer close properly. Sometimes they get caught under the hook. The latter can often be fixed.
A needle with a damaged transfer spring can knit fine but will act up when transferring. Push the needles up to have a clear view of the springs.
A needle with a broken butt can’t be selected by the carriage and won’t knit. This is easily recognized by looking at the aligned butts.