You can see in the IDE under MyDevices the status of a device and the Last Activity. But the new mobile app can also tell you this now. If you have the Device Health enabled it is suppose to flag you when a device is no longer available. In you mobile app you can check this under the More, Additional Features heading. You can do a search here in the forums and some people turn this off because of other issues but I can’t remember why.
The GE Link bulb should be OK, I have about 10 of these and a couple things to consider is when someone flips off the light it can’t be repeating and they do drop off the network from time to time and need to be reset by flipping the light switch 5 times , on then off every two seconds.
Do a search here in the forums on the Lowe’s Iris smartplug. I heard that it is supposed to not only do Zigbee but also Zwave. I got one of those for my house to boost both. But it’s the newer version that does it not the first generation I think.
In the IDE you can also watch the devices Logging to see if its passing information.