Since it's a private method I wouldn't worry about verification within it. That should be ensured before the method gets called, where ever the value comes from. Since you're likely getting the value from an external source - public methods parameter, or a file, or another db call - definitely validate it there.
As for testing it, if you have complete coverage of your public methods that call this, it'll get covered (and any paths inside it as well). If it isn't getting correctly covered, then either you don't have full coverage on your public methods or it's not getting used and scrap it.