When Does Impurity Transfer
You do not need to investigate the impurity. If there is no taste, smell, or colour of impurity, take it pure. Deen is easy and does not oblige people to bother themselves unnecessarily. Still, you think the impurity is there, wash it away.
When you are not sure, then you are not responsible for letting anything impire stay impure. You can get rid of impiriy only when you are sure it is impure. In case of impiriy do not use your imagination. Imagination has no limit. Our religious doesn’t like making things hard on you unnecessarily. Deen is easy. If for example an impurity went unnoticed, you will not be responsible for that. And your prayers etc will be accepted.