Is it necessary to change underwear for every prayer due to discharge, the purity or impurity of which cannot be distinguished?

There is no obligation to inspect for any discharge. If secretions appear and their purity is not clear, it is permissible to consider them vaginal moisture, which is pure but nullifies ablution.

However, if it becomes clear that it is madhiy (pre-seminal fluid), then it is impure and nullifies ablution, and whatever it has touched on the body and clothes must be purified. If it becomes clear that it is maniy (seminal fluid), then it is pure according to the preponderant opinion, and ghusl (definition) (ritual bath) is required without needing to purify the clothes. The matter of purity is made easy in Islamic law.