I know that you can get all kinds of lists illustrating the endless signs and symptoms for overtraing, so I wont just cut and paste. I will speak from a practical standpoint.
As a trainer, I think it is very important to remember the “personal” part of our title. For me, I start wonder about overtraing in my clients when their dedication and desire to workout starts to dwindle. I think there are obvious physiological signs but a lot of times we won’t see or hear about those from our clients, its normally when their performance will start to decline or they start missing appointments more than usual that get me to investigate overtraing.
Hope this helps,