Examine your surroundings. Observe. Think.
Are you comfortable or uncomfortable? Who looks right and who looks suspect?
Scan for any obvious physical hazards.
What can you improvise as a weapon?
When you enter a room, taxi, airplanes, train, anywhere, at all times, see the exits: hollow core or steel doors, lighted stairways or not, escalators, elevators, windows with fire escapes or windows with a deadly drop.
"I'm in. Now, how would I get out?"
What could block your reaching an exit? How long to reach that exit?
If trouble erupts, who looks like they'd panic and who could be relied upon to help?
In the beginning, awareness takes conscious effort. Quickly, awareness will become a valuable instinctual skill, a habit.
Give your all, limited only by circumstances beyond your control.