Purity: Obligation to Examine for Obstructions
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 292:
If a person doubts whether or not something is stuck to a part of the body on which wuḍūʾ is performed, in the event that his deeming this probable would be considered by people to be reasonable – for example, after plastering or painting one doubts whether or not plaster or paint is stuck to his hands – he must examine that part or rub his hands to the extent that it would normally give one confidence that it has been removed or water has gone underneath it.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)