There are a lot of tools that can help measure the pH of a substance. But in this case, the most convenient tool should be the pH paper. It is a piece of small strip of paper that will change color according to the substance to test pH.
If the coffee makes the pH paper turn orange or even red, it mean the coffee is an acid, it has a pH of lower than 7. By the way, there should be a indicator scale on the box of the pH paper, so that you know what color represents what pH. The lower the pH is, the more acidic it is. 7 is neutral, and below 7 is acid, and above 7 is base.
If the color changes to blue or purple, the coffee would be a base.