Make sure you have one intermediate and one heavy sinking head (type 3 and type 6) for different water types. Here is what I use to determine (and systematically change) the depth I am fishing at. Pick a head that you think will get you down. Unless the water is very low or I am fishing a very small stream, I start out with a type 5/6. Cast 45 degrees up stream and make a mend if you need to (careful though..mend too much and it will be difficult to detect a strike). I typically find bottom sometime during this inital drift (hopefully early in the drift). By incrementally changing the angle of your cast downstream you can adjust your drift to run the fly at shallower depths, because your line will begin to swing under tension sooner (shortening the time the sinking head can "free-fall"). This will allow you to run at a slightly different range of depths just by changing your presentation angle. Use a lighter head in the same spot and you can cover another range of depths. Ideally you can then cover from bottom to top if you need to (tip...fish high or low (don't bother fishing middle water). In winter, deeper drifts generally work better.

Good Luck!