I’ve never had to deal with a federal internal checkpoint, but it could happen anywhere in New Hampshire or Massachusetts, and they are set up now and then in New Hampshire’s tourist areas, so it’s important to be prepared. I don’t have practical experience other than actual border crossings, and I’m no lawyer, so I’m linking to hopefully reliable sources here rather than offering my own advice.
The ACLU has a reasonable-looking page on “What To Do If You’re Stopped By Police, Immigration Agents or the FBI.” Key points are that you have the right to remain silent (but you should say out loud that you’re exercising that right) and that you have the right to refuse a search. I’ve turned that page into an EPUB file and put it on my iPod. Ken White at Popehat advises that not saying anything is very often the wisest thing to do. An innocent error or an attempt to cover up what you think is none of the government’s business can result in your being prosecuted for making false statements. The first question may be innocuous, but it’s better to stop there than to balk at “Are you running guns to terrorists?”
At an internal checkpoint, it appears from several web pages I’ve seen that the best approach is this: Decline to answer any questions, no matter how many times they’re repeated. Ask, “Am I being detained?” If not, then ask, “Am I free to go?” If you don’t get a straight answer, keep asking. If you’re asked to pull over for an inspection, you do have to pull over, but you don’t have to consent to a search. Ask, “Do you have a search warrant for my vehicle?” and don’t accept any substitute for one. If they pull a gun on you or physically assault you, though, don’t be an idiot; it’s no longer a question of law but of facing an armed and dangerous person with lots of backup. In doing all this, it’s best to maintain an attitude of calm confidence in your rights and not be belligerent about it. Stories I’ve seen indicate that this will almost always get you waved through after a while. Please note, this is neither legal advice nor advice from my personal experience. It is the approach I plan to take if I ever have to, based on my current knowledge.
This doesn’t apply if you’re actually crossing the border. There, you’re basically screwed if they want to screw you, though the odds are in favor of nothing happening. The best approach there seems to be to have a simple explanation of your trip planned out in advance, to be able to say exactly where you’re going, and to let the driver do all the talking. Avoid saying anything that sounds as if you’re crossing the border for paying work. Mentioning music often rouses suspicions that you’re planning to use drugs and do paying gigs, but making up a story to avoid mentioning it may be more dangerous. (If you are doing a paying gig, you need better advice than this.) This paragraph is from personal experience, but nothing more than that.
Having a lawyer’s phone number readily at hand is a good safety measure and can increase your confidence when you’re at a checkpoint. If you think you’re being racially profiled, you can call the ACLU profiling hotline at 855-737-7386.