Well, it depends on the situation, but if you're looking at corporations, if one corporation controls the other corporation, then they're considered not to be at arm's length, or if they're controlled by the same person or are part of a group of companies that are controlled by the same person, then they would not be at arm's length.
If they're not related, if they're corporations owned by different people, then it's a factual question, just as the test in the Income Tax Act is just a factual test, a question of fact, on whether they deal at arm's length. Depending on the situation, there would be all different facts that might be indicative of the fact that people are not dealing at arm's length.