Testing CBD/THC ratios before flowering?

How is the testing method called?
Which part a the plant is used as sample for this?

Is it the same test used for cannabinoid rates on final buds?
(HPLC?)

Im told early testing is unreliable. I can't remember where I heard this but some plants will show high cbd in veg via testing of the leaves and no cbd whatsoever at the end of the flowering cycle. anyone can confirm this^?