In the summer, they will usually be just above the thermocline, which could be as much as 70 feet down. In the other 3 seasons, when water temps are colder, they will orient to drops and other structure, making them easier to catch. Put yourself on a drop off and suspend your bait half way between the surface and the bottom, or wherever your fish finder is displaying schools of bait fish. Trout in lakes are moving almost constantly, rainbows even more than browns, and tend to come through in schools. Rainbows will usually hit the bait on a dead run, but browns sometimes like to play with it a while, so this is one instance when you should let the fish run a bit before setting the hook.
Slip bobbers are also useful for stream steelheading.
Floating your bait just above the bottom means far fewer snags and, because the line enters a slip bobber from the top, it's easier to present a more natural drift. Pinch the smallest one on about a foot above your bait and the largest one on about 6 inches below your float. Space the other 2 evenly between the first 2, with the larger one nearer the float. If this all sounds backwards, there's a good reason behind it. River current is faster at the surface than it is at the bottom.