How to tell whether a slope is positive negative zero or undefined?

When the gradient of the slope is negative or positive, the slope is negative or positive respectively.
or if u wanna tell by looking at the straight line graph, then if it makes an acute angle with the x-axis it's positive, if it makes an obtuse angle with x-axis its negative.
the slope is zero when the graph is horizontal
then slope is undefined when eh graph is vertical.
hope this helps