I feel stupid saying this but when I ride the only thing I can see is my horses head, and I know that just looking at his head and neck is slightly counter productive as a good head/neck set doesn't really indicate working from behind, but I don't have an indoor with mirrors so I can only see his head.
For instance today when I rode I was asking for more from Phoenix, more of anything really (he's so lazy and ploddy most of the time) and when he actually offered a bit more speed I asked him to slow down with a half halt and he slowed down and sort of nodded his head down and felt more bouncy. I was doing rising trot so I couldn't tell whether his back was coming up into my seat (i wish I was better at sitting trot so I could feel more). He stayed like this for a few strides and then went a little flat again, like someone had deflated him.
Also while i'm asking questions. How can I get my horse to be a bit more energetic? Normally I don't mind his laziness but sometimes it just takes way too much effort from me to get anything out of him.