There are some clear indicators that you can use : - number of warheads active - number of anti-ballistic measures in play - active conflicts involving nucle

There are some clear indicators that you can use : - number of warheads active - number of anti-ballistic measures in play - active conflicts involving nuclear armed powers - active conflicts between nuclear armed powers - new nuclear technologies (ie. Bayesian fuses, hyper-sonic maneuverability, suppressed trajectories) - new nuclear powers/capabilities (ie. NK gets H-bomb, NK and India deploy boomers) - change in treaties (withdrawals) - threats made by national leaders I don't know what you mean by "definition of safety" - while there are thousands of nuclear weapons in play no one is actually safe, ever.