It seems obvious to me, that real time tests that can gauge the approximate time of consumption are the only logical solution. Just determining that someone used marijuana(illegal, or not) in the last 30 days or so, doesn't pass the smell test when it comes to determining impairment at the time of a recent event.
Unlike others in this thread, I don't believe the Federal Justice Department will spend the time and funds to go after individual users, especially in states that have decriminalized it. President Obama has said as much. He said that the law is the law though and those who operate on a large scale will still be prosecuted. So we may see some fireworks on that front, if and when big growers/distributors in Colorado, or Oregon are prosecuted.
"No free man shall ever be debarred the use of arms."- Thomas Jefferson