I often just open them up and rub my finger on the rim of the jar-mouth, get a little on the finger tip, and taste it. You need only a tiny amount. If I don't like the flavor that way, a
juice is not generally going to enjoyable...and even if I do like the flavor that way, the final
juice might not be great. Its funny how something that is really great when eaten can be rather yicky when vaped a while. Lorann coffee is one, so is kiwi-strawberry, at least to me.
Also, when I first use a flavor for a new
juice, I generally start at 10%, then if its not strong enough, add a few more drops. I only make 3 mL tops for a first time juice. If I find myself going through all 3 mL, that recipe is generally a keeper.
Oh, and Scubabatdan's ejuice calculator spreadsheet is a must for me. I use syringes for accurate amounts (sometimes I'll add a few drops of a flavor, as I mentioned) and safety (I make my juices from 100 mg VG unflavored).
Good luck!