When there is a vertical asymptote, either it approaches that asymptote going to infinity or negative infinity. So pick a point just to the left of the asymptote, and see if it is positive or negative. Then do it for the right (although you'll quickly learn you don't have to do it for both. You'll see what I mean).

So find f(3.999), f(4.001), f(-3.999), and f(-4.001).