Trichomes are often called crystals because they start clear and reflect light. As they age, the resin in the trichome head becomes cloudy. When most of the trichomes are cloudy, you can start sampling every few days. Most growers harvest when 10% to 50% of trichomes have turned amber.(This often depends on the grower's experience with the strains involved. Some are better at a particular stage.)
Get a magnifier( jeweler's loupe or microscope) with a 30x or stronger magnification to see the trichomes, clearly. Trichomes are NOT "hairs". Most people refer to the pistils as hairs. Especially once they are dry.