1. The question asks: How do we know if a function or value is constant? 2. A constant means the value does not change regardless of the input or variable. 3. For example, if a function $f(x)$ always equals the same number, say $5$, for every $x$, then $f(x) = 5$ is constant. 4. Mathematically, a function $f(x)$ is constant if its derivative is zero everywhere: $$f'(x) = 0$$ 5. This means the slope of the function is zero, so the function does not increase or decrease. 6. If you have an expression or value and it does not depend on any variable, it is constant by definition. 7. In summary, we know something is constant if it does not change with respect to the variable or input, or if its derivative is zero.