Comment by cryptonector

The use of nuclear weapons would be... obvious: if an explosion in the 10Ktn or bigger happens, it's a nuclear weapon. There aren't enough nuclear powers to make the use of nuclear weapons plausibly deniable.

The use of chemical weapons might not be as obvious if they are slow acting. And the production of chemical weapons is much easier than that of nuclear weapons. Though, the dispersion of chemical weapons is non-trivial.

The use of biological weapons need not be obvious at all -- "it's a naturally-evolved pathogen, this happens!". The development and production of biological weapons is much easier than that of nuclear weapons. Human and animal bodies can be made to help spread biological weapons, so their dispersion can be trivial. The only thing that a bioweapons user might need ahead of time is treatment / vaccines, unless the bioweapon is weak and the real weapon is psychological.

Sobering thoughts.