Answer: By looking at its behavior on the graph.
Explanation:
You can tell if a zero (sometimes referred to as a solution or an x-intercept) has an even or odd multiplicity by its behavior in the graph.
If it has an even multiplicity, it will appear to bounce off of the x-axis. If it has an odd multiplicity, it will cross through the x-axis.
You can see attached for a graphed example of this.