Verbal checks are absolutely fine - in fact, it's verbal action that's binding so if a player said check then put some chips in the pot, their action would be a check.
The other thing to keep in mind, I guess, is when you're watching poker on TV you're only seeing a select few hands. While it's true that some players do have some different, "non-standard" ways of checking, you'll usually find that those players are at least consistent in the checking motion they use. So the guy that taps his own arm to check is probably doing that every time he checks.
If that's the case then I don't think it's such a big deal, as the dealer and other players who are sitting at the table hand after hand know that player tapping his arm means check.