sheesh quite an art without a microscope. The pistils hairs should be turning brown towards late flowering. Certain cameras are good enough to take a high quality picture and zoom in on the trichomes. My growing partner has a Huawei p20 pro and that camera is so good it can zoom in on a single trichome and the colour be identified. Maybe see what cameras are around with good zoom function? Besides that its kind of hard to detect without the correct tools as they are so small...
Happy growing pal