Robyn, you're looking for dressings that are made primarily from good fats (like olive oil, canola oil, or soybean oil), are low in saturated fat (hopefully have none...) and have 3 grams of sugar or less (3, 2, 1, or 0 grams of sugar are all acceptable). Of course, the best dressings have no "high fructose corn syrup" and are lower in fat (less than 11g of fat is great, IMHO).
Check your dressing for this, and see how it goes.
If you want to post the nut'l info, we can let you know what we think.