How to validate a credit card number with your head using a checksum as presented on mint.com's blog. Take the above number (or any credit card number). And double every other digit from the right. Add these new digits to undoubled ones. All double digit numbers are added as a sum of their digits, so 14 becomes 1+4. If the final sum is divisible by 10, the number is invalid or fake. Try it and see.

How to validate a credit card number (click for full pic)

[via:Viralliset Linkit]