Purity: Investigating Obstructions in Tayammum
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 699:
If a person deems it probable that there is an obstruction on his forehead, palms, or the back of his hands, then in the event that his deeming it probable would be considered by people to be reasonable, he must look into this until he becomes certain or confident that there is no obstruction.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)