I recently activated some nematodes and didn't have time to apply them. I put the mixture in the refrigerator and waited until later to apply them. How long will the nematode mixture last in the refrigerator? How can you tell if they're still alive?

I don't know how long they will survive since they need something to eat after being activated; I suspect not too long. I would get them out in your yard ASAP. To see if they are alive, use a cheap microscope or a strong magnifying glass. They should be wiggling around. If they are not moving at all, they are dead.