There is a couple websites in my high risk niche that is processing credit cards and I would like to know if there is a way to find out who they use for processing if not advertised. I can PM the sites If anyone can help happy to pay.
Short answer is no, you can't. But you can do a few different things to get closer to the information.
You can try enabling developer tools in your browser and monitor the HTTP requests to see which URLs are being connected. But that might not tell you much, if they are using a payment gateway with many different providers.
Another is to make a payment and look at how it appears on your bank statement. Does it say the URL/company you bought from, or something else?