Where does air in the fuel come from

I have a rough idle, if a leak is comming from "While your running, air can be sucked into the fuel system from a failed "O" ring on the tank connection or the selector valve", would this be visible in a clear line running from the return rail up to the filter head?

Thanks

great, I read though the post for clear line test but wasn't clear wether it would indicate where the air is comming from.

Thanks for the help!