BTC buying OTC afraid of flash btc

so i ve heard of flash btc (fake btc). it had happened to my friend before when he bought BTC with cash.

I am buying BTC tomorrow from OTC office. I want to get a 1 satoshi test but the seller insists on 1 BTC test (says minimum for test is 1 btc)

i will be getting the BTC to my wallet, checking, then giving the cash to the seller.

question is how can i know its not flash? how can i make sure?? does flash btc not get confirmed on blockchain explorers?

so that means if the transaction is confirmed the btc is real?