How do you know if someone calls you using a pre-paid phone card?

If someone callse you how can you find out if the number shows up on caller ID is from a pre-paid phone card??

How do you know if someone calls you using a pre-paid phone card?
There is no way to know for certain. However, many calling cards will appear to come from some caller ID that may or may not be real and this can be a clue.