That is where your meter becomes your best friend, you know by testing. With my girl I always get a test right before I leave the house just to check on her and again right before I go to bed for the night.
It is caused when their bloodsugar drops too low, and you can't always tell by looking until they are very very low. Musette has several times been acting perfectly fine and I have tested her to find her in the 30s. So I get her food out and get her to eat, now after months of testing her I know her patterns very well, I also know that she will spike on even low carb food so unless she is in the 20s I don't break out the high carb food. But that is Musette others will be showing signs and need High carb sooner.
As you go along and gather data from random spot checks you will learn Casey's patterns. You will know when she is dropping too fast or if she will spike on food as well as when her onset (when the insulin starts taking effect) as well as when her lowest point is likely to hit. It just comes with lots of testing and watching for patterns.
When I first adopted Musette even though I already had Maxwell and knew what I was doing with a diabetic cat I hated to leave her too, but once I had gathered a lot of data on her I just randomly spot check her when I know I going to be gone and leave food down for her, because she has proven that she will eat on her own if she thinks she is dropping too fast or going to low.
Mel, Maxwell, Musette & The Fur Gang