Does one have to check a cloth before using it or is it only advisable? Many times I grab one from the bag and forget to pre inspect them. Does it make a bedika invalid if you don’t look at the cloth first? What if you didn’t see anything questionable anyway after the fact?
There is no need to check the bedika clothes that we buy nowadays, because they are pre-checked and considered clean. There are however poskim that recommend checking it beforehand for practical reasons, because there are times that there can be minute colored fibers, which can be mistaken as blood, therefore they recommend checking it beforehand. If it wasn’t checked the bedika is still fine. If you don’t see anything on the cloth after making the bedika then the bedika is surely valid, as nothing was found on it.
Shiurei Shevet Halevi pg. 189, Poskim.