Assalamualikum my question is that my carpet became impure of a specific impurity but after the impurity dried i don't remember which spot on the carpet is impure what do i have to do now?
What you have to do is clear if there's a risk of impurity transfer because you use this carpet extensively then you should wash the carpet or replace it.
Note that a carpet you don't use in the means of a hypothetical impurity transfer doesn't need to be cleaned at all.
However imam as-Suyuti quoted in his book on fiqh rules al-Ashbaah wa an-Nadhaair كتاب الأشباه والنظائر:
قاعدة: " قال القمولي في الجواهر: النجس إذا لاقي شيئاً طاهراً وهما جافان لا ينجسه
Rule: “Al-Qamuli said in Al-Jawaher: The impure (najis), if it met (touched/mixed with) something pure (tahir) and they are dry, it does not defile it.
This means a dried impurity doesn't transfer by default!
The main issue as said before is a Muslim must guard or seek tahrah (ritual purity) in order to be able to pray. So even if najasah was on your wet feet all you have to do in order to pray (in first place) is being sure you washed your feet before starting the prayer.
As in the moment of prayer both you and the place you are praying on should be tahir (no more no less).
However washing off the najasah from the carpet would give you peace of mind for the future.
However the matter of impurity transfer in such cases is rather exaggerated because little najasah is equal to no najasah.
Further impurity has at least one of the three characteristics color, odor and taste. You may locate it by searching for them using your senses. But if you fail there's still the option of total certainty: washing the whole carpet.