The test is to graph it and then slide a vertical line along that graph. If the vertical line intersects the graph of f(x)=-x at most once then the equation is a function. The key here is that for each possible input x, there is one corresponding value of f(x). Here is a graph of f(x)=-x with a vertical line for illustration, sliding the line left or right you can see it will only intersect f(x)=-x once:
