If you expect that it's rotted, you haven't really forgotten it. That's the whole point: they expected empty containers. Which could happen to you, being (presumably) a human.
I can tell from the weight and feel of the container if it's empty.
Thermos bottles are relatively heavier to their contents than flimsier bottles. That goes double if they're only half full. I have a couple of the recalled bottles, so I just weighed one - 14 ounces empty.