How to detect the value of a variable is stagnant or not?

How to detect the value of a variable is stagnant or not?

Hello,
I receive a variable of an equipment by ZenOn supervision. Normally the value of this variable changes every second. If its value doesn't change during a long time, for example one minute, that means the equipment meet some problems.

So, my question is: How to easily detect a variable value is stagnant or not? If it is stagnant, I want to create an alarm.

Thanks a lot at advance!

This is a migrated post! Originally posted on 04.03.2010 by user lephis. Please be aware that information can be outdated.