Most of the missile tests to date have been done from Kwajelin (sp). Those tests are planned for months in advance. Not only aren't the missiles ready to go operational, I don't think they'd be in range to hit a missile launched from North Korea.

Some tests have been performed successfully using Aegis cruisers. Those can be deployed along the probable flight paths and would have a chance at a successful intercept. That might be a last resort, but I'd still try it in a pinch.

Other than that, there is no missile defense system.

But, to be honest, the chances that they'd actually launch are quite small. What they want is to use nuclear blackmail to force us to the bargaining table. They can threaten to launch but about the only chance they'd do it was in a death spasm as their country collapsed under the dead weight of a failed system.