Getting better at it, we use the sounder religiously when jigging for kings. The first thing to do is to turn off the fish symbols, it becomes easier to work out what is down there. It will take time, but after several months I can now tell the difference between a school of yakkas, a school of wrasse and kingfish. Part of this has involved catching whatever is down there.
We also use the sounder to find structure, and cruise around it to see if fish show on the screen before we anchor up and berley when chasing snapper.
Realize this is not lake fishing, but I reckon same principles apply.
The more you use it, the easier it gets to target the species you are chasing, they each have characteristics that set them apart, ie snapper seem to school at different depths where wrasse all seem to hang at the same depth.