You can buy testing kits, or there's a method at the beginning of the DIY section, sticky I think.
But in general the nicotine is neglible, and IMO you need to add it separately, after extraction, before vaping.
This may, however, depend on the extraction method, and products that you use. Somebody else may have a better answer???

Up to now, I've only done it by cold VG/PG maceration, and the Nic content came from a bottle of Nicotine, not at all, or very very slightly, from the tobacco.
As far as I know you need certain solvents and chemist's knowledge to extract nicotine from tobacco. BUT certain people say that it can be extracted in water, so see what others say, as I'm not a chemist.

Would be cheaper if it disolved in water

but I have doubts...