How to Tell if Plants & Shrubs are Dying

I have several indoor and outdoor plants/shrubs that I don't know if they are dead. I know this sounds odd, but here's why:
I have two 3 foot Yellow Ribbon Arborvitaes that I planted about a month ago in our front landscape bed outdoors. These shrubs have lost some of their yellowish color and appear limp and dry. After inspecting the inner leaves up and down near the main stem, I noticed that there are many browned and dead leaves that can be easily pulled off. The outer leaves seem green with a ever so slight tinge of yellow and with the exception of the look I describe above, they do appear "alive".
I also have an indoor Cactus that is in a small pot and never grew above a foot high. It appears alive but it has accumulated two yellow patches about an inch in diameter on some of it's shell.
I continue to water and care for both described plants but I don't know if either is really alive and how to tell if they are dying. Can you direct me to a web site that explains what to look for?
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