If while performing wudhoo’, i do not know whether the wetness on my body part is from the wetness of my hand that rubbed onto me or the wetness is from washing, then do i follow the principle that i have not washed until i am certain that i have, or do i assume that the wetness was from flowing water?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu 'alayhi wa sallam, is His slave and Messenger.

If you are suffering from Waswasah (obsessive satanic whisperings), then you should ignore such whisperings and pay no attention to them. If you are not suffering from Waswasah and your body was already wet before performing ablution and you then were unsure whether the water reached the wet body part during ablution or it was already wet before it, then the basic principle is that water did not reach it. Therefore, you should wash it until you are certain or mostly believe that water has reached it thoroughly.

Allah knows best.