[jameshart]

One answer to that is to apply property based testing. If you're testing an addition algorithm, for example, you might test that

   A + B == B + A
   A + 0 == A
   (A + B) + C == A + (B + C)
   A + B != A + C (given B != C)

[slavik81]

It's a good idea in principle, though your third and fourth properties are not true of floating point addition.

When you have one available, testing against the results against a reference implementation is a little more direct. Though, given how easy those property tests are, you might as well do both.