End to end continuity for each of the conductors (including the extra signal cable). Then you have some options to test for IR. You can do LN->E and LS->E for example.
Thinking about it, after two more sigs of beer, the LS-E isn't going to prove much as the signal switch won't be closed on the loop through the alarms, so that's just a daft idea as you'd have to do it at each location.
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