If I am understanding this question correctly you want to know how to test the nicotine content of a premade juice. Usually a titration test would be done on nicotine solution and not an already mixed e-juice to determine the nicotine content. The flavoring and color of the mixture will effect the outcome of the results. I don't think there is a way to test this.
If your talking about unflavored extracted nicotine solution then you need to test with a titration test kit such as this one.
Test Kits and Reagents
If your question is how do you calculate the amount of concentrated nicotine to add to a juice your making for instance (how much 100mg nicotine do I add to 30ml's of base to make it 12mg). Then you would use an e-juice calculator like the one below to calculate that for you.
eJuice Me Up - Best eJuice Calculator
Please make sure you know exactly what your doing before working with NIC
If your extracting your own NIC be very careful because your walking down a very dangerous road.
Look below for further information.
My blogs will give insight into all aspects of DYI, from steeping to what supplies to have on hand to a basic walk through mixing your first E-juice.
E-Cigarette Forum - dannyv45 - Blogs
Then read hoosier's blogs. These blogs concentrate on fine tuning your mix and give insight on additives.
E-Cigarette Forum - Hoosier - Blogs
Then read Boletus's blog's. These blogs concentrate on formulation of nicotine bases regarding proper calculations methods for figuring Nicotine concentration as well as safety.
E-Cigarette Forum - Boletus - Blogs
Then read Ginger's book "E-Cigarettes 102: DIY E-Liquid". It is a book which I've contributed to that delivers an in depth easy to follow guide in the art of DYI.
E-Cigarettes 102: DIY E-Liquid
I recommend viewing VPLive Vape Team Episode #82: DIY Safety and bookmark it for future reference. It explains safety in all aspects of DYI.
VPLive Vape Team Episode #82: DIY Safety
then read the stickies at the top of the page.
this will explain a lot of the how's and why's.