The best way to test Datavalidation is to intentionally cause an error with a wrong value and check if the exception matches what you expect, and then to put in a valid value and check that no error is thrown.
The difficulty with it can be, if a field has alot of validations and you want to test the validation errors individually you'll either have to find a value that only causes one certain exception which can be difficult in alot of cases or ignore every other error except the one you expect.
ORCA has its own Datavalidation Testing plugin which I do also sometimes work on teehee.