How can I tell which credit cards my checkout is setup to accept?

I got a note from a client that said she couldn't use any card but Visa. Where do I double check the credit cards that I selected during setup for our checkout system to process. We have Paypal Payments Pro.

On the checkout page, there is a drop down list of credit card types. the checkout wizard will set up the list to include the following card types:
Visa
Master Card
Discover
American Express
Switch
Solo

ok, they're all there but someone trying to make a purchase said she tried American Express and MasterCard but it wouldn't take them. The credit cards are on the list of course as an option so I just didn't know why they wouldn't work unless it was a problem with her own credit cards. I'll assume based on your answer then that if they show on the dropdown, the system is set up to accept them. There have been some orders with MasterCard, I couldn't find any with American Express but I did notice almost all were with Visa, so I was inclined to believe her...for a moment.

eCart only passes the details to the gateway, the gateway collects the credit card details that are passed in and tries to process the transaction. It really has nothing to do with how eCart is configured, the success or failure on processing the card happens on the gateway side.