You should have a general idea of how much hair usually remains behind in your comb, shower or pillow after sleeping. When you can clearly see there is more than the usual daily amount, it may be an indication of excessive hair loss. Another indication would be to tug on your hair and it falls out easily. Thinning hair can also be identified when there is less hair to clip or tie back than before. For a diagnosis, consult a hair care professional / medical practitioner.