Raising kids has, no doubt, made you realize that they can tote a lot of stress. Sometimes, out of duty, kids will mask that stress-load from moms and dads to be a "good soldier." The last thing that they want to do is make you worry about them, even though they are truly struggling. Here is a good read from Tim Elmore about was to spot stress in your kid and what to do about it.
10 Visible Signs of Stress and How to Spot Them