Increasing/Decreasing
With a Table
With an Equation
You cannot tell for sure that a function is increasing or decreasing over an interval, but you can tell for sure if it isn't increasing or decreasing.
Given only the equation of a function, sometimes we can apply our understanding of transformation of functions to know where that function will increase vs. decrease, but for the most part doing increasing vs. decreasing with just an equation is a calculus topic, not pre-calculus.