You can easily determine which is which after you pull the tank and then put some tape or a mark of some sort on one of them so you can tell them apart once they are installed. If you need to know before you pull the tank, you'll need to draw down the fuel in the tank until it is below the reserve tube that you can see sticking up inside that tank below the gas cap. Check to see which line isn't drawing fuel and that's your main line. The reserve draws from the fuel left below the tube. I have pulled a couple of fuel tanks and never had much trouble routing the lines through the frame. You shouldn't need fishing line. I just pushed them through.

Doug

Doug - Olympia - Washington
 I don't know how many CTs I have.