Checking maturity of trichomes is the only right way. For me it looks like she has a month inside, to
@Roberts it looks a week... Everything is relative. It depends on your outdoors season too, sometimes girls go overdrive. Last year I couldn't manage to chop one girl, because she was still packing flowers, not bulking even. And it was middle of November in UK, we had frost in the nights and she was still pushing, had to use her for Bublé hash.
Beware of pests and mold, check girl daily, hope she will reward you LOADS !!!
Cheapest scoop/magnifying glass x60 or so times - cost up to 10£ max. Invest and do it right !!!