While this seems like a simple question, there are no simple answers. Many methods have been devised and tried, but even counting individual hairs is not reliable, since healthy hair goes through a phase where it falls out then regrows months later.
So, just because you see hairs on your pillow or in your sink, it does not mean you are suffering from permanent hair loss. The truth is, there is no easy, reliable way to monitor your hair loss. All you can do is be aware of your overall density, how easy or difficult it is to see your scalp through your hair, where your hairline rests and how all of that changes over time. It is too easy to be paranoid and obsessive about hair loss, and all of that stress is surely not helpful.
If you are losing hair due to an illness, stress or disease, you may see rapid hair loss, hair falling out in clumps or bald spots appearing on your head and should consult with a physician. Rapid hair loss can result from serious underlying factors and should always be investigated with your doctor.
Our best suggestion to monitor your hair is to take standardized photos of yourself every 6 to 8 months and compare them. Set up your camera so you can duplicate the same position, lighting and pose when you retake your photos.