On dual tanks when replacing fuel pump, how do you know which one is bad

Most likely. The fuel running through the fuel pump motor cools and lubricates the pump motor. When the gas tank runs dry the fuel pump motor can burn out very quickly. Hope that helps! Jim

I had 87 silverado dual tanks and truck quit running. Told both tanks burnt out. I told mechanic what was the odds, then he found out it had some sort of valve under cab of truck that switches from left tank to right tank. I think fords are on same side at least filling doors but check that out 1st.Still had to pay almost 300 to have them put pumps and tanks back in because neither where burnt. Also check button on dash where my electronic electronic valve routed from. Good Luck

Its called diagnosis. Get a mechanic to help you.