How do you know whether a slope is positive or negative when there's no number line?

If it points upwards, it has a positive slope. (goes up from left to right)
If it points downwards, it has a negative slope. (goes down from left to right)
If it is perfectly horizontal, its slope is 0.
If it is perfectly vertical, its slope is undefined.

You can always count on your independant variable (x-axis) being horizontal and your dependant variable (y-axis) being vertical, so these do not change.

As you walk the graph from left to right, you would climb a positive slope, or slide down a negative one.

However, if the workers do outside of what's normally expected, go ahead and tip.