check if an always increasing variable has stop increasing

Generally, the easiest way to compare is to maintain a second variable with the value at the previous point of reference.

 // C#
 
 float lastValue;
 float currentValue;
 
 void Update()
 {
     currentValue = valueToMaintain; // Whatever value you might be holding
     
     if(lastValue == currentValue)
     {
         // The value didn't change in the last frame
     }
     
     lastValue = currentValue;
 }

Can't you just store its value from 1 update to another, and check if the new value is bigger or not?