Firstly: Whoever is Doubtful of Some Water's Purity or its Najasah
Whoever is doubtful  Scholars of jurisprudence use the word shakk (doubt) to refer to unsurety in the existence or non-existence of a thing, regardless of whether both sides are equal in doubt or one is more likely than the other. over some water's state of purity, whether it is pure or najis, then he builds upon the original state. If he is certain that the water was pure then doubts whether it became najis, then he treats it as pure as that is the original state wherein there is certainty, and he may use it accordingly. If he is certain of some water's najasah and is doubtful as to whether or not it became pure, then he treats it as najis, as that is the original state wherein there is certainly, and he mustn't use it. This is by agreement of the four schools of jurisprudence.  The Hanafis may put forward water that is most likely pure due to accompanying evidence.
Secondly: Whoever is Uncertain between Purifying and Najis Water  A possible scenario to highlight the context: some vessels contain purifying water while others contain najis water and one is unsure which vessels contain which water. Where the scholars differed: 1) If one has no purifying water for certain; 2) If one is unable to purify one water with the other.
Whoever is uncertain between purifying and najis water should seek clarity and investigate,  Investigation is the seeking of truth and searching for what is desired. then purify himself with what is most likely the purifying water. This is the position of the Shafi`is,  Shafi`is went further than this, which is the permissibility if using water that is uncertain in its purifying ability, even if able to use water that is certainly purifying, like if one is at a river or if mixing the waters would reach more than two Qulla (a specified quantity by jurists above which water does not become najis). and it is the verdict of Dawud al-Zahiri, Abu Thawr, Suhnun the known Maliki scholar, and it is the choice of Ibn al-`Arabi and Ibn `Uthaymin.
Thirdly: The Ruling of a Just Trustworthy Testimony Regarding Some Water Being Najis, Giving a Reason for its Najasah
The testimony of a just, trustworthy source regarding the najasah of some water is accepted if the reason for the najasah is highlighted. This is by agreement of the four schools of jurisprudence.
Fourthly: The Ruling of a Just Trustworthy Testimony Regarding Some Water Being Najis, without Giving a Reason for its Najasah
The testimony of a just, trustworthy source regarding the najasah of some water is not to be necessarily accepted if there is no reason given for the najasah. This is the position of the majority: Malikis, Shafi`is, and Hanbalis.
Fifthly: If there is Uncertainty with Regards to Some Clothes, the Pure, Najis, and Haram among Them
If there is uncertainty with regards to some clothes, the pure among them and the najis or haram, like if it is stolen or forcefully acquired, then investigation should be undertaken and clarity sought. One would then pray with one piece of clothing most likely to be pure. This is the position of the Hanafis, Shafi`is, an opinion among the Malikis, chosen by Ibn `Aqil al-Hanbali, Ibn Taymiyyah, and Ibn `Uthaymin, and al-Qadi Abu al-Tayyib transmitted it from most scholars.