The proof is in the pomegranate...
Real grenadine is made with the juice from pomegranate seeds (the French call the funky red fruit "grenades," hence the name). It's quite easy to mix up a homemade batch--and much tastier too. And if you add an ounce or so of vodka as a preservative, the concoction will last for a month or longer, refrigerated
It's pomegranate season right now, so if you find picking out the seeds to be Zen-like (like I do...) have at it, and then crush them to yield fresh juice. Or you can take an easier route and purchase pomegranate juice from POM or other producers (available in most grocery stores in the refrigerated section by the produce.) Depending on which you use, you may need to adjust sugar levels to taste. Here is an easy recipe for grenadine I found on Epicurious from Audrey Saunders, owner of the Pegu Club in NYC:
Courtesy of Audrey Saunders, Pegu Club, New York
6 ounces unsweetened pomegranate juice, such as Elite Naturel
3 ounces superfine sugar
Bring juice to simmer over medium heat and cook until reduced by half, about 7 minutes. Reduce heat and add sugar, stirring constantly until dissolved, about 2 minutes. Let cool, cover, and refrigerate syrup until cold, about 30 minutes.