Maybe? Only one way to find out really.

Decommission them all; then you will know for sure.

The only winning move is not to play.

If nobody is left does that count as finding out? If a tree falls in the woods...

It is also a bit of a strange comparison since the weapons produced today are hundreds of times more powerful than the ones produced sixty years ago. It seems like the US or Russia will prominently decommission old bombs and people will hail the lower number of nuclear weapons but in reality the total amount of destructive force in the nuclear arsenal long term is the same or greater.

Weapon yields are much, much smaller nowadays than in the 1950s and 60s.

The highest-yield US warhead was the 25Mt Mk 41, retired in 1976. All warheads in the inventory now are in the sub-Mt range, mostly around 150 to 300 Kt

Back then ICMBs were accurate to several miles, necessitating large bombs. Today they are accurate to meters. Not only you could do with smaller bombs, but it's feasible to attack many more targets. The destruction potential of today's nuclear arsenal is significantly larger than it used to be, although it uses smaller bombs.

how do you even prove that

Several inspections and basic statistics.