What should be done to verify the source of colorless wetness observed on clothes upon waking, in order to avoid the whisperings of major ritual impurity (janābah) and its effect on prayer and Quran recitation?

If you are certain that the wetness you found upon waking is manī (seminal fluid), then ghusl (ritual bath) is obligatory upon you. However, if you are not certain that it is manī, then ghusl is not obligatory upon you, and you may choose at that point between what you are doubtful about. Beware of waswās (whispers/obsessive thoughts) and do not doubt the validity of your prayer or abandon reciting the Qur'an.