Since zwave is a mesh network, signal quality has to be measured someone differently, but in general, you’re looking for three fields:
LQI
RSSI
and number of transmission failures
Tried checking the new advanced page in the official smartthings web interface at:
http://my.SmartThings.com/advanced
If it’s not there, there is a Community-created project which has some additional information, and it might be in that, I’m not sure:
SmartThings API Browser+ ... Now Available to All
(I was a Z wave engineer before I ever got my first SmartThings equipment, but I no longer use zwave with smartthings so I’m not sure of the current available details.)
Otherwise, hopefully someone else will comment.