Flintlocks were guns that used round bullets as their ammo and for this ammo to be activated, they made use of a crucial material called gun powder.
As its name might propose, gun powder was a flammable material that when ignited propels the bullet to leave the gun and hit a target.
To anyone that had fired a gun or perhaps even heard about, the basic knowledge of gun powder and how necessary it was to fire arm was a well-known thing, and just as one would guess, for gunpowder to work, it would have to be in the good conditions, one of those conditions being that it must be dry.
With this common knowledge on flintlock and gun powder, it made perfect sense to the woman with the rifle that with how soaked Kruxe's pistol was, whatever powder had been put in it was already wet and the fact that it was still under the rain and getting beaten by it further strengthened and solidified her conclusion.