with putting, unlike approach shots, it is all about feel for me .. & practice, as other posters have mentioned ..
my method is this:
- i try read 1st half of put & judge where the ball should be 50% on its way to the hole
- then i try judge whether the remaining 50% of the put will take the ball to the hole or not
- if not, i adjust the start-off line & repeat the process
after some time this process becomes reasonably quick ..
i also always use the larger scales for putting & not the scale the dumb-a$$ caddy hands me .. i.e. for 5ft put i use 50ft scale .. easy to ding compared to the 10ft scale ..