well you know how deep you are with say a jig, t-rig, shakey head, etc., because you're going off the bottom with these presentations and can feel the bottom as you work the lure (hopefully). With your other sinking lures, unless you're working those close to the bottom, as well, you may want to try figuring out the sinking rate of whatever lure you're throwing. That way, you can make a cast and "count down" the lure until it reaches the depth you wanted to. Say you find a lure to sink at about 1foot per second and you want to suspend your lure in 8 feet of water. make your cast and count to 8 when the lure hits the water. at 8 seconds, the lure should be at the desired depth.