The family of B vitamins are vital in the bodies stress response. They are also an essential input in the Krebs cycle - the chemical reaction in your body that produces energy.
Therefore if you're stressed you'll use up your B vitamins more quickly and so find it harder to generate energy.
For me the tell tail signs I'm low in Bs is a general lower energy level and fatigue/reduced stamina when exercising.
B vitamins are also important for skin health so dry skin and flaking or split nails can also be a sign - I tend to get dry circles on my upper arms when I'm stressed which is my prompt to have the brilliant Nutri Muscleze supplement which is rich in Bs and Magnesium.
Food wise you need to eat plenty of brown rice and other wholegrains, leafy greens, oily fish and eggs, so if these aren't a regular part of your diet they should be, especially when you're going through a stressful time.