I go a lot by smell in pot (though this can be deceiving sometimes with good teas) and the taste of previous infusion (factoring in rough time of previous infusion too)
With most japanese greens I find there is a distinctive 'done' smell as well.
How much the tea has opened also gives some hints.
And of course your previous experiences with the tea should give you a good idea of how far it will go.
I am totally out of japanese greens
Holding out till next week when what I want arrives in stock at yuuki.
Some 98 8582 this morning. Delicious. 1999/3 Xia Guan Jia-Ji Toucha last night. Yum. Thank you houde!
Now some light roast dong dong at work.
Come on where my darjeeling at!