How do I check the radiator fans to see if they are working properly?

What is your question?

I've had the same check engine light as so many have. I thought I had it fixed when I replaced the gas cap but after it warmed up and was sitting at a redlibht, it came back on. After a four hour road trip, I looked under the hood to see if the fans were running while it was idling and they weren't. It was a cold day but I shought they should be going anyway. What can I do to check.