9.3. The marker may only start and continue a stall count when:
9.3.1. The disc is live (unless specified otherwise);
By 8.1.2, the disc is dead:
After the pull or after a turnover when the disc must be carried to the location of the correct pivot point, until a pivot is established
So the disc becomes live when they reach the pivot point.
You may stall as soon they establish a pivot in the correct place. After an OB pull or OB turnover, there exists a correct place where they should establish a pivot, even though the disc is currently elsewhere, and once their foot reaches that point you may initiate the stall count.
It’s possible that there may be some disagreement about the exact location where the pivot should be established, but this can be covered by the travel rule (if the thrower establishes the pivot in what the defence considers to be the wrong place) and can usually be avoided if the defender indicates in advance (by pointing or similar) where they expect the pivot to be established.
The offence is not permitted to reach the pivot point and then ‘evaluate the offence’. You may start the stall count.
Benji