In our location, it's easy. If armed people are northbound on the Interstate Bridge, from Portland,
DROP 'EM. They are baddies. Besides, they won't have the proper verbal challenge and response, or the radio freqs. They don't have the phone trees, the proper tac codes on their cars. Oh, didn't I mention that we've been planning for this for years? Long since distributed, and tested.
If they are southbound on I-5, from Olympia, they are hostile. DROP 'EM. There are any number of choke points. Units up there are likewise equipped, and coordinated with us.