This could be the true verification that this technology needs. In the past, targets were launched by us and downed by us. Now, if we down a target put up by a moronic, unpredictable dork, we can truly say we have a winner.
Seriously though, this is a good test of the radars and crews. They'll have to be on alert starting, well, just about any time now. They won't get notice of specific, or even general launch times. To add to the fun, no real idea of the actual flight path - what azimuth and elevation it'll be fired at, nor exact staging and range. Finally, no telling if it'll be a "clean" missile or dump a lot of debris on staging events...maybe even toss out a few decoys or chaff if it really is an IRBM test. So the radars, alert crews, and the whole intel side of the house are going to get a workout.
As for reaching Hawaii or Alaska, that really depends on total impulse in the booster stack and payload weight. They might be able to loft a lightweight test into orbit - similar to the Iranian satellite. That was what, 50 or 60 lbs? However, if you put a ton or more of warhead/re-entry vehicle on it, and it will not go as far.
A ton or so is probably a fair estimate for a first generation warhead/vehicle. They'll have to over-engineer it some to ensure it works. These things have to survive launch stresses (more than 5Gs is a good guess), atmospheric heating and aerodynamic forces (pressure and vibration). Then there's the cold of space and exo-atmospheric solar radiation. Depending on re-entry angle and vehicle aerodynamics, they can smack the upper atmosphere and catch huge G loads. Then there's tremendous friction heating and more vibration. Inside is the physics package where you have to maintain precision alignments and all the components need to work with nearly flawless timing...
What I'd like to know is this... Where are the NK "fishing trawlers?" If there are some suspicious NK surface vessels anywhere out in the Pacific, that might be a potential re-entry area. If they're testing an IRBM, they'll no-doubt instrument the heck out of the boosters and the dummy payload. They'll need intel ships nearby to collect the signals.
Anyway, if this thing gets anywhere near Hawaii, even if it looks to make orbit, I think we ought to shoot it down. It would be a good system validation. Who's going to know? Who has radar coverage around Hawaii but us? Think anyone would notice if USS Lake Erie paints another satellite silhouette on the side of the wheelhouse?