Depends on how you have this setup. If you have rules setup to store the value from devices into variables, you could use a separate rule which triggers off of the variables changing and compares them.
So lets say your first rule is just taking the Humidity and storing it into a variable:
Note that I used the Context Variable: Event → Device → Value here which makes this rule execute lightning fast as it just stores the triggering value directly rather than trying to look it up from SmartThings
Then in your second rule, you could use that variable changing as a trigger.