Ok, here's the scenario. 2 players play in a heads up poker game with family and friends for Player 1's birthday. Flop has 4, A, Q. Player 1 (P1) goes all in. Player 2 (P2) takes a long time deciding to call. P2 flips over 1 of his cards and reveals an Ace. P2 says out loud, "if I didn't have a low card to accompany this Ace Id quickly call but I don't know...this is tough." P1 doesn't say anything. P2 then turns over second card, revealing his hand completely to be A,2. Still has not said if he is going to call the All-in bet. P1 then says his hand is dead. P1 thinks you cant reveal your cards before you decide to call or not. P2 says that in tournament settings this is valid. And after seeing P1 argue, P2 calls the bet. The turn and river are flipped, no straight for P1, and P2 wins the pot and eliminates P1.
No rules of the cash game at home were determined before starting. Nobody thought to really address it because its not a professional tournament.
Can you show your hand in a cash game heads up poker game before calling a bet?