Some men are excessively paranoid about hair loss. Others may be in denial, and so do not accept their own hair loss for several years. Either, it can be difficult to objectively identify hair loss, especially because baldness tends to occur very slowly.
Generally, though, you or someone else will notice that your hairline has gradually receded or that your hair has thinned. You may also notice that hair is coming out in the shower or on your pillow.