This can be understood by drawing on a graph. However it can also be done by hit-and-trial/option elimination :
- If $p$ lies between infinity and $0,$ suppose $p$ is $-10$, then $p-1=-11.$ Product is greater than $0$
- $p$ lies between $0$ and $1,$ suppose $p$ is $0.9$ and $p-1= -(.1)$ $\text{less than 0}$
- Let $p=10$ then $p-1=9$ product is greater than $0$
- Let $p=8, p-1=7$ Product is greater than $0$