There are only two ways I know of. One is just having social skills. You should be able to tell when someone isn't happy; you should notice when someone doesn'

There are only two ways I know of. One is just having social skills. You should be able to tell when someone isn't happy; you should notice when someone doesn't fully engage in a debate in which they should; and when they don't meet your eye about something. The other way is to look at the details of how they behave. This is how I notice when I myself am not motivated about something. I notice that sometimes I still "feel" motivated and cheerful, but I start turning up a little late. And there's always a good reason for it -- but looking at the pattern from the outside, there's a hint something is wrong.