How can I verify the integrity of smoke detector interconnect wires?
First I would check the wiring to the smoke detectors. You should have three wires, one white (neutral), black (hot) and another color usually red (the interconnect between the smoke detectors). Then I would verify that they are all pigtailed and are going to each individual smoke detectors. While I had them down I would also check the batteries and make sure they are charged. May just replace them to be sure. If everything checks out you should be able to get all of them to go of by pushing the test buttons. First the one you pushed the test button on then after a slight delay the others should start alarming. 

If you test them and each one is sounding locally, then the interconnect is not connected between the detectors.

After you test them and one is not alarming, I would say it's probably bad and needs replacing.