The standard the Prophet, peace be upon him, gave
A man brought exactly this doubt to the Prophet, peace be upon him, in the middle of prayer. Notice what he was given. Not a method for becoming sure. A threshold. A sound, or a smell. Something he could actually verify. Anything short of that does not count.
Qur’an and Sunnah
لَا يُكَلِّفُ ٱللَّهُ نَفۡسًا إِلَّا وُسۡعَهَاۚ لَهَا مَا كَسَبَتۡ وَعَلَيۡهَا مَا ٱكۡتَسَبَتۡۗ رَبَّنَا لَا تُؤَاخِذۡنَآ إِن نَّسِينَآ أَوۡ أَخۡطَأۡنَاۚ رَبَّنَا وَلَا تَحۡمِلۡ عَلَيۡنَآ إِصۡرࣰ ا كَمَا حَمَلۡتَهُۥ عَلَى ٱلَّذِينَ مِن قَبۡلِنَاۚ رَبَّنَا وَلَا تُحَمِّلۡنَا مَا لَا طَاقَةَ لَنَا بِهِۦۖ وَٱعۡفُ عَنَّا وَٱغۡفِرۡ لَنَا وَٱرۡحَمۡنَآۚ أَنتَ مَوۡلَىٰنَا فَٱنصُرۡنَا عَلَى ٱلۡقَوۡمِ ٱلۡكَٰفِرِينَ
God does not burden any soul with more than it can bear: each gains whatever good it has done, and suffers its bad- ‘ Lord, do not take us to task if we forget or make mistakes. Lord, do not burden us as You burdened those before us. Lord, do not burden us with more than we have strength to bear. Pardon us, forgive us, and have mercy on us. You are our Protector, so help us against the disbelievers.’
Qur’an 2:286
Abbad b. Tamim reported from his uncle that a person made a complaint to the Apostle (ﷺ) that he entertained (doubt) as it something had happened to him breaking his ablution. He (the Holy Prophet) said:He should not return (from prayer) unless he hears a sound or perceives a smell (of passing wind). Abu Bakr and Zuhair b. Harb have pointed out in their narrations that it was 'Abdullah b. Zaid
Sahih Muslim 361graded sound, sahihaynfull report
Translation: M.A.S. Abdel Haleem, The Qur'an, Oxford University Press, 2004.