This is an evolving area of law and it's an area I deal with often. Our Supreme Court has determined that general criminal control checkpoints are unconstitutional. However, they have also determined that DUI checkpoints can, under certain circumstances be Constitutional. Also, checkpoints at an International border in most cases are Constitutional, as well as internal checkpoints, if they are for immigration purposes.
But, keep in mind, anything an officer discovers, either by you telling him or by sight or smell, they can then take action. Also, keep in mind, since most of these checkpoints are Federal checkpoints, so if you have a medical marijuana card, they do not have to honor that.
Many people have asked me, "do I have to answer their questions." To this, I say, "it depends." You can, of course, refuse to answer all questions, but be prepared to be detained. If you just would rather be on your way, it is probably in your best interest to just answer the questions and be on your way. You need to ask yourself, "do you want to be an activist or just get to where you are going?"
My advice is, you should just answer the questions and be on your way. You live in a Police State, and there are certain transaction fees for living in a Police State.