Ask questions before purchasing. If a vendor can't or won't give you the answers you need to feel confident about buying their liquids, buy from a different vendor. I'm 100% confident and satisfied with the equipment and methods my preferred juice vendor uses - he lists the ingredients on his website, will freely discuss how he mixes his juice and even where he obtains his ingredients. Basically, he'll tell you anything you want to know short of disclosing his actual recipes.
I don't consider buying e-juice any differently than I do anything else - I do my research and make up my mind that it's what I want before I lay my money down.