I shot two rolls of Presto today, one @400, the other pushed to 1600. I forgot to label which one was which (I know, I'm still kicking myself.) The way I see it, I have three options
1. Dev them both @400 so at least 1 roll comes out (D-76 for 7mins)
2. Dev them both @1600 so at least 1 roll comes out (D-76 for 14mins)
3. Dev them in D-76 for about 10 minutes ... would that work or ruin them both?
How about developing one of them then deciding what to do with the other? You'll have a 50% chance of getting the first one right and a 100% chance of getting the second one right...
Another option is to develop one roll (you have a 50:50 chance of being right) and then developing the next accordingly. You could also decide if one roll is significantly more valuable to you than the other.
BTW: Even a pencil will mark film canisters well enough. You could even scratch them with a key or a coin.
I'm surprised no one has suggested a clip test - Surely you could run a clip test for ISO 400 on one roll? If it's properly developed, that's your 400 roll, if it's dim, it's the 1600.
I cut off about 2" of a roll and did it in D-76 for 7:30. Doh!! The 2" piece had two photos I really wanted to keep ... The negs look okay though, and at least now I know which roll is which (and to bring a #$%& pencil with me next time.)