Recidivus gives good advice about buying just small bottles, samples, or sample packs of juices to try out from different vendors, until you find a flavor you like enough to decide to order again in a larger quantity.
If you drip, its relatively easy to test these juices out. Myself, I never acquired the knack for it. Instead of wasting an entire carto to test each individual test flavor, I began using the newer ce3 clearomizers. I use the smaller ones that are less than 1 ml in capacity. It doesn't take much juice to decide if you like it or not. These are easily filled with a small needle and syringe. They are clear so you can see the juice to note its viscosity. They use a wicking string, no polyfiller to alter taste. Inexpensive and relatively reusable.
If you don't care for the juice, withdraw it with the syringe, rinse the ce3 with water, and refill with the next flavor. You can find these in either the clearomizer or cartomizer section of vendor websites.