There are three ways to establish the impurity of an object: 1. one is certain, or is confident by rational means, that the object is impure. If one only supposes (i.e. has a ẓann) that an object is impure, it is not necessary for him to avoid it and it is ruled to be pure.
Therefore, there is no problem in eating in public places, restaurants, and guesthouses where the people who eat there are unconcerned about religious matters and who do not observe laws relating to what is pure and what is impure, as long as one is not confident that the food brought to him is impure; 2. someone who is in possession of an object says it is impure; for example, one’s spouse or domestic worker says that a utensil or something else that they have in their possession is impure; 3.
two dutiful men say that an object is impure, on condition that they give the reason for its impurity; for example, they say that the object has come into contact with blood or urine. If one dutiful man, or another reliable person, says something is impure but one does not attain confidence in what he says, the obligatory precaution is that one must avoid that thing [i.e. it is ruled to be impure].