Tap off a bit, add some table sugar and let it sit at 70F for 3-4 days. If there is still enough yeast, you'll know quickly.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk