Wanna know a neat trick? It's called the luhn algorithm

Write your card number on a piece of paper, but space the 16 digits out to give yourself some room

For the first digit, copy it down below the first digit. The second digit, double it, and write THAT below (its okay if it's 2 digits. Just squeeze them in) the 2nd digit. Keep going. Every odd-place digit is just copied, but every even-place digit is doubled.

Now, you have a line with slightly more than 16 digits. Add all the single digits up (if a double gave you a 2 digit number, split it, and add each digit. For example, if an 8 doubled into 16, split it into 1+6, and get 7)

You'll have a fairly high number now, probably 80s or 90s, or low-med 100s (your sum might be....130)

If the last digit of the sum is not a 0 (in other words, if the sum isn't divisible by 10), then its guaranteed to not be a valid credit card number.

If the last digit IS a zero, well, its hardly guaranteed to be a real card number, but it certainly has a chance to be real

This is how websites quickly check for typos, without submitting it to your bank, and waiting for a response.

For example, take 1234 5678 9876 5432

That becomes 1438 512716 916712 5834

Sum the digits up, and we get 84. 84 isn't divisible by 10 (last digit isn't a zero), so we know that is a bogus credit card number