Ive only played texas holdem with actual people twice in my life. The last time i was playing there was a man sitting next to me that when he held his 2 cards to look at them i could easily glance down and see what he was holding because he was very careless. I didnt say anything to him but i saw at least 5 of his hands. I was actually kind of scared the dealer would see me looking and accuse me of cheating so i didnt keep looking. Should I have told him what he was doing? Should I have just kept on peeking at his hand? And is it cheating for me to see his hand even though I didnt move my head to see his cards, all i had to do was look down?
Yes, you are ethically required to tell the player and/or dealer that you can see someone's cards. Usually a quick and polite "Protect your hand, sir, you're giving me a peek." will suffice. I've given second warnings as well, but after that, well, the player is presumably an adult, and he's been warned.
Ideally, you should notify the dealer that you can see the other player's hand. If someone were able to prove that you were able to see the other player's hand it may be considered evidence of Collusion. In reality, it's easy to deny that you saw the other player's hand, so you are free to keep looking as long as you are not obvious about it. It's each player's responsibility to protect their hand.