cc test (click here)

There is the Luhn formula that is a mathematical way of seeing if a creditcard number is real, and it goes something like this:
1. Double the value of alternating digits
2. Add the separate digits of all these results
3. Add the unaffected digits
4. Add the results up
5. If this final sum can be evenly divided by 10, we're good to go.

Anyway, click on the above to test this formula in a little javascript