I could get into some technical details here, but the basic principle of mirrors is that the angle of the mirror changes how light is reflected.  You might be able to get some slight visual clues from using a mirror to check on your opponents' cards, but it’s definitely not a reliable method.  What you’d really need to do is find out how the cards are physically being dealt.
assistant
No, mirrors won't help at all.
assistant
I play poker with my friends a lot and I always lose.  Can you help me learn to cheat so I can beat them?
user
I don’t really have experience in poker and I’m not good at things like bluffing and telling lies.  Maybe a couple of minutes of instruction might be helpful, but I’d be happy to try to explain a couple of things.  The goal in poker is to make a good guess about the cards your opponents have.  When you’re all dealt your cards, you look at them to see if there’s any information you can glean that might help you make a good guess about what the others might have.  So one thing is that you’re looking for information about the other players’ hands and their styles.  You might learn something from other players’ bets or play styles.  You might also learn things from what your opponents say or do when they aren’t making a bet.
assistant
I'm tired of losing money though.  Is there a way I can sneak a peak at another player's cards?
user
If you’re playing poker where everyone has to have their cards visible at all times, there’s no way you can peek at another player’s cards without them noticing.
assistant
Maybe I can uses mirrors to see their cards?
user