Move your thumb.
I always thought this was common knowledge, but apparently not, so I'll share this factoid with you.
Credit cards tell you what kind/brand they are by the 1st digit.
3 - American Express starts with a 3.
4 - Visa starts with a 4.
5 - Mastercard (formerly Mastercharge) starts with a 5.
6 - Discover card starts with a 6.
(Learn more about what the numbers mean.)
I first realized this when I would give my number to a company only to hear, "Is that a Visa or Mastercard?" I wanted to say, "Move your thumb," but I found myself educating the person with the information I've shared above.
Now you know, and knowing is half the battle.