This isn't rocket science.

Fill both tanks. Leave the handle pointed in one direction. Drive it for a week. The tank with the missing gas is the one the handle is pointed at.

I had this set up on my '73. When you fill up you need to switch tanks. Otherwise the gas in the un-used tank will get stale after a while.

My Fleet:
1941
1953
1959
1965
1966
1953 Willy's Pickup

John

Vegetarian- old Indian word for bad hunter