Short answer
Treat it as pure. In Islamic law everything is pure until you know it became impure, and certainty is not removed by doubt. You do not need to check, smell or ask. Wash only what you are sure is najis. The four schools agree. If doubts keep coming, that is waswasa, and the cure is to ignore it.
Things are pure by default. Purity known with certainty is not lifted by doubt, and impurity is only established by certainty or strong evidence such as sight or smell. A person with frequent doubts (waswasa) disregards them entirely.
The four schools agree on this.
Explanation
A man told the Prophet ﷺ he felt something during prayer and did not know whether he had broken wudu. He said not to leave until you hear a sound or notice a smell. The scholars built a rule from this that covers all of purity: certainty is not removed by doubt. What you know to be pure stays pure until you know otherwise.
Everyday cases:
- A wet patch on a public toilet floor or seat: treat it as water.
- Clothes from a shop, a laundromat or a non-Muslim's home: pure.
- Your child's hands, the playground, the bus seat: pure unless you see impurity.
- Food from a restaurant kitchen: pure for this purpose. Whether it is halal is a separate question.
- A stain you cannot identify: pure. Wash it if you like, but you do not have to.
- You know a drop of urine fell on your clothes, but not where: wash the area where it most likely fell, and then stop thinking about it.
- You find impurity after praying and do not know when it got there: do not repeat past prayers on a guess. Ask a mufti if it was large.
Allah has not placed hardship on you in the religion. Checking your clothes again and again, washing things three or four times "to be safe", or avoiding places in case they are dirty is not piety. It is the waswasa (whispering doubt) that the scholars tell you to ignore. If it has a strong hold on you, it may be OCD, and treatment is part of taking the means. See waswasa about wudu.
For what to do when you are sure, see cleaning najis from clothes and how much is excused.
Ask a mufti about your own case. This answer explains the general ruling. When the details of your situation matter, as they do here, ask a qualified mufti and give them the full facts. Where to ask a scholar.
The evidence
Quran, Surah Al-Hajj 22:78
وَجَٰهِدُوا۟ فِى ٱللَّهِ حَقَّ جِهَادِهِۦ ۚ هُوَ ٱجْتَبَىٰكُمْ وَمَا جَعَلَ عَلَيْكُمْ فِى ٱلدِّينِ مِنْ حَرَجٍۢ ۚ مِّلَّةَ أَبِيكُمْ إِبْرَٰهِيمَ ۚ هُوَ سَمَّىٰكُمُ ٱلْمُسْلِمِينَ مِن قَبْلُ وَفِى هَٰذَا لِيَكُونَ ٱلرَّسُولُ شَهِيدًا عَلَيْكُمْ وَتَكُونُوا۟ شُهَدَآءَ عَلَى ٱلنَّاسِ ۚ فَأَقِيمُوا۟ ٱلصَّلَوٰةَ وَءَاتُوا۟ ٱلزَّكَوٰةَ وَٱعْتَصِمُوا۟ بِٱللَّهِ هُوَ مَوْلَىٰكُمْ ۖ فَنِعْمَ ٱلْمَوْلَىٰ وَنِعْمَ ٱلنَّصِيرُ ﴿٧٨﴾
And strive for Allah with the striving due to Him. He has chosen you and has not placed upon you in the religion any difficulty. [It is] the religion of your father, Abraham. Allah named you "Muslims" before [in former scriptures] and in this [revelation] that the Messenger may be a witness over you and you may be witnesses over the people. So establish prayer and give zakah and hold fast to Allah. He is your protector; and excellent is the protector, and excellent is the helper.
The religion is not meant to burden people with constant doubt.
Sahih Muslim 362
Sahih... اذا وجد احدكم في بطنه شيئا فاشكل عليه ...
"If one of you feels something in his stomach and is unsure whether anything came out."
The basis of the rule that certainty is not removed by doubt.
Where the ruling is recorded
- al-Ashbah wa al-Naza'ir (Ibn Nujaym), The second principle: certainty is not removed by doubt. The Hanafi statement of the principle and its applications in purity.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Taharah, chapter on najasat. Things are presumed pure; the waswasa sufferer ignores doubts.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.