I do a lot of float fishing for steelhead, a good rule of thumb is if you see your bobber ticking or stopping mid drift a lot you are dragging bottom. Adjust a couple inches shallower, and drift again. Keep in mind though that with most fish you want your offering running very close to the bottom, so unfortunately although it's a pain, the guys who snag bottom the most also usually catch the most fish. If you can pinpoint that perfect depth near the bottom without dragging, your catch ratios will increase significantly