A plateau in performance.

A drop in performance.

Elevated Resting HR (an easy way to measure this is to take your resting HR when you first awaken and compare it from week to week).

Elevated Training HR (if you know at level 6 on the treadmill you normally have a heart rate of 120 bpm, and lately has been measuring 150 bpm, something may be wrong).

Feeling of "heaviness."

Ongoing muscle soreness (chronic).

A desire to skip workouts (your body is telling you something and you should listen).

Lack of enthusiasm when it comes to both the gym and everyday activities.

Decreased concentration.

Tiredness.

Sleep disorder (both too much and not enough).

Lack of appetite.

Weight loss (when not trying).