It's not real easy. It's a lot of work. Obviously the more time you have, the better chances of finding schools. But, like me, most of us don't get that much prefishing time. So you have to pick seasonal, high percentage areas before you even leave for the lake.
For instance, this last trip for me at LOZ I knew a lot of fish had spawned out. So I figured the best, most active schools would be closer to the main lake. Sure enough, that was the case. Secondary points and transitional banks inside the coves didn't have the schools. But inside main lake transitional points, they were schooled up.
Unfortunately, that is not always the case. Sometimes they are not doing what you think they should be doing. And that means you will not always find schooling fish. So I will change areas and presentations in hopes of finding schools somewhere. It is a gamble with limited prefishing, but it has high rewards. If I fail to find them, then I can always resort to the areas that will have one here and one there.
To answer your question specifically about how to find schools, pick your high percentage areas and fish only around them. Don't go running down a nothing bank just because it is adjacent to your ledge, creek channel, etc. Fish the high percentage area with different crankbaits, jigs, finesse, topwater or whatever your confident baits are. Use different retrieves, cadence and speed. Check the top of a break first for active fish. Then try the break itself. Finally, don't leave until you have targeted the inactive, suspended fish. That is why it is important to have more than a few rods ready to go. Drop the crankin stick and pick up the jig. Drop that one and throw jerkbait. Drop that and let a wacky rig fall through the water column. If no bites, then move on to the next area you have picked out.
Here is a final tip. Always fish a lake as if it is your first time there. In other words, do not just fish your past spots. Your productive spots may not fit the conditions and you will waist time. That doesn't mean that you shouldn't check them. But if they aren't productive, go to the areas that match your conditions.