Nicotine concentration in e-liquids
Nicotine concentration is a serious concern, as the material is a serious poison (dose matters).  Liquid Chromatography/Tandem Mass Spectrometry is a little pricey.

http://oxfordchemserve.com/e-cigarette-refill-nicotine-quantitative-test-kit/ 
  Specific test kit "designed with the non-chemist in mind" 
http://oxfordchemserve.com/wp-content/uploads/products_img/Oxford_Nicotine_Test.pdf 
   Explanation - it is a pH titration of nicotine base. 
http://www.eliquidtest.com/ 
   Another titration kit.  Google for the product spectrum.

Saliva or urine nicotine test kits detect nicotine persistent metabolite cotinine and are therefore irrelevant.

http://www.medpagetoday.com/Pulmonology/Smoking/44238 
Warning about E-liquid refills.  
http://www.veppocig.com/product_images/uploaded_images/eliquid-nicotine-strength-guide-2.png 
Claimed dosage guide.  Caveat emptor. 
http://www.learn.eversmoke.com/nicotine-strength.html 
A second dosage opinion.

Glycerin can pyrolyze to acrolein.  This is not a good thing to inhale.  NEVER inhale butter flavor.  Butanedione destroys lungs: "popcorn lung," bronchiolitis obliterans, obliterative bronchiolitis, constrictive bronchiolitis are life-threatenting, cumulative, and irreversible.