This is the test I use, and you can assemble your own kit for about $20.
The Reason:
Since there are no true production standards, and people like to mix their own liquids, this will allow you to test liquid for nicotine levels easily.
The Kit:
.12N Sulfuric Acid
Bromothymol Blue
Distilled Water
10mL Graduated Cylinder, .1mL markings is optimum
A paper towel
The Test:
Pour some distilled water in the cylinder, swirl it, dump it out.
Repeat step 1.
Repeat step 1.
Dry cylinder with a sliver of paper towel.
Put 1mL of eLiquid in the cylinder
Add 2 drops blue
Fill cylinder to the 3mL mark with distilled water
Swirl to mix
Add a drop of acid
Swirl to mix
If the color of the solution didn't turn pale yellow, repeat 8-9
Measure the final volume
Subtract 3 from the number you read in step 12
Multiply the result of step 13 by 19.4712
The resulting number is the mg/mL
Notes on measuring: Always measure from the center of the liquid, where its at its lowest point. Never measure from the high point on the sides.
Notes on adding to the cylinder: Avoid dribbling liquids down the sides, as it will skew results, since some will add to the amount when you swirl to mix. For an accurate end result, exactly 1mL eliquid must be used.
Note on step 10: Over several drops, the color will go from blue to bluish-green to green, and when you've reached the 'magic drop', it will suddenly all turn yellow.
Note on final result: If the manufacturer added acids for preservatives, it should not skew the results enough to matter, as they use very minute amounts. Also, as most DIY'ers can't read volumes to several decimals, bear in mind there is a small margin of error. I tested 12 different bottles, 10 of the results were within 1mg of their rated, all just slightly low, probably due to flavoring being added.