Question

explain how you can just look at an equation or an expression and correctly identify which type of change it represents, growth or decay.

Answer 1

Assuming you can graph it out, you would always have a gradient at certain points.
A positive gradient = growth
A negative gradient = decay
If it's in the form y = mx + c (linear), m would be the gradient. Polynomials would need to be differentiated to find the gradient.