Detect if a key is bound to something in vim

I'd like to know if there is a way to figure out if a key does something in vim. I know that I can use :map to see user-defined mappings, but is there something for the built-in stuff?

For example, I always had CTRL-W bound to close tab, because I thought that it was unused. After half a year, I found out that there are some sequences that use it, like CTRL-W CTRL-S to split the window, and it was a nightmare to retrain myself.