Let's start with a link. The Nationwide Suspicious Activity Reporting Initiative can be viewed by clicking here: http://nsi.ncirc.gov/ Federal, state and local law enforcement officials have set up “fusion centers” for the program in about a dozen cities, including Boston, Chicago and Houston, where reports of suspicious activities made by citizens and the local police are collected and analyzed for disturbing patterns.
There is no real way to discern someone who is nervous to someone who is suspicious. As we are learning in Boston, even the most jovial and well-liked people can cause harm and do horrible things. The best policy is to adopt the one New York uses. "See something, say something." Boston was able to identify its suspect because of people seeing something and taking photos. These photos led to the identification of their suspect. So, if you feel in your gut that your neighbor, co-worker, or whomever needs to be checked out- have them checked out. It is always best to be safe than sorry.
As a Homeland Security expert, I am aware of different internal tactics used to weed out potential threats. However, there is no public rule for weeding out or verifying someone's suspicious activity. So, when in doubt- have it checked out.
Stay safe out there,