In some cases they can be interchangeable. But generally I would use verification to confirm that an answer or outcome was correct. e.g. "The test showed there were no side effects, but we showed the results to some other people for verification". Validation would be more used in regard to the method used. "The results were positive, but we still need validation of the tests that were applied." Validation is also used more generally, and in more personal contexts: "I said he couldn't be trusted, and now that he's gone to jail I feel validated!"