Unfortunately, there's only one easy way to tell which tank is 1 and which is 2. You have to fill up one, either the toilet or sink, and see which one registers. It was only recently when they decided to define each.
I suppose if you were handy with electrical stuff, you could always try to power up one of the lines from the tanks and see which one it will register for.
There was nothing ever in an owners manual that described which was which.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR