Pay attention to the state of your voice when you finish singing. Speak in a normal tone. If your voice sounds or feels hoarse, this is a sign of strain and you should begin to rest your voice.
Sing a warm up song and note how well your voice hits the high points. Consistent difficulty singing high notes you are accustomed to indicates signs of fatigue.
Touch your throat and check for tenderness. A voice that has experienced overuse or abuse will swell and feel sore.
Watch for a itching sensation in your throat and excessive dryness. This is a sign of a poorly hydrated throat. Treat this by consuming fluids and taking a break from singing until your voice feels better.