Three indicators, in order of usefulness:

Trichomes (most reliable). Under a 30–60x loupe: clear = early, milky/cloudy = peak THC, amber = THC degrading to CBN. Most growers target 70–90% milky with 5–15% amber for balanced effect. More amber = more sedative; less amber = more cerebral.

Pistils. White and upright = early flower. Curled, darkened (orange/red/brown), and tucking back into the bud = approaching ripeness. Aim for ~70%+ pistil maturation as a coarse signal — but trust trichomes over pistils.

Breeder notes. Most reputable breeders publish a target flowering window (e.g. 56–63 days). Use this as your starting search window — but verify with trichomes.

Different lines peak at different windows. Sativa-leaning lines often want 50–70% milky for clean head effect; indica-leaning lines tolerate more amber for couch-lock. Reading the strain is part of the harvest.

I don't have a loupe — what's plan B?+
A pocket microscope or phone clip macro lens are both under $20. Pistil-only timing works for casual growers but loses 1–2 weeks of precision.