The sniff test would probably work, I am guessing anyway.
Funny, but my platoon sergeant, many moons ago, was a former Marine and served in Vietnam. Anyway.... we were about 3 days into a 2-week field exercise. I was complaining that we didn't have the use of latrines... we had to dig "cat holes" with our shovels to take a dump. Which, mind you, is a lot easier in the summer months when the ground is soft versus the cold months when the ground is frozen. You'd find a tree (that you could lean up against), dig a hole (hopefully avoiding the roots) and that was your makeshift toilet for #2. Then you'd do the necessary paperwork, pull up your trousers, then go about covering up the hole you just dug and took a crap in.
Anyway, one particular field exercise we had a shortage of toilet paper. My platoon sergeant addressed it the following morning, Day 2 of our field exercise.
"I was given three single-ply sheets of toilet paper in 'Nam," he said, "with a toothpick." Of course, I was the first to ask what the toothpick was for.
His reply: "To clean the shit out from underneath your fingernails. So deal with it."
So when we used to joke about turning our underwear inside-out to get another 2-3 days of use out of them, we weren't necessarily joking.
The life of a grunt in the field back in those days....