How can you tell the difference in Visa and Mastercard numbers?

I found t his link that may help: http://www.merriampark.com/anatomycc.htm

The two leading credit card companies in the world today are the competitors Visa and MasterCard. They both operate along very similar lines. While Visa can claim to have almost a billion cards...

There is not much difference and there is a LOT of difference! A visa number is assigned to a person entering the USA. It later can become his/her Resident Alien number if alien status is...

"There are several ways to tell if a credit card number is a Visa card number or not. First, Visa numbers always start with the number 4. They are also usually 13 or 16 digits in length."

There is no difference in the two, it's just another way to write it