When can we consider an object touched by a kaafir as najis?

We have a general rule that everything is tahir unless known to be najis

Najasat transfers through moisture and not a dry thing touching dry thing.

Cleaning this najasat of kafir requires only cleaning once with water.

About transferring of najasat see the following rule of Ayatollah Sayed Sistani:

Ruling 119. If a pure object touches an impure object and both or one of them is wet – such that the wetness of one transfers onto the other – the pure object also becomes impure; however, it does not become impure through multiple intermediaries [i.e. the transfer of impurity is limited to two intermediaries].
An example: if the right hand has become impure (mutanajjis) with urine, and [after drying,] that hand touches the left hand with a new wetness, this touching causes the left hand to become impure; and if after drying, the left hand touches some wet clothing, for example, the clothing also becomes impure; but, if the clothing touches some other wet object, that other object is not ruled as being impure. Furthermore, if the wetness is so little that it does not transfer onto another object, the object that was pure does not become impure even if it touches an intrinsic impurity.

Ruling 120. If a pure object touches an impure object and one doubts whether or not both or one of them was wet, the pure object does not become impure.