jsharlan,
100SX? That’s an old one for sure. Hard to believe now-a-days that a 128 by 64 pixel count display was cutting edge for sonar units at one time. You should start by downloading the operations manual if you don’t have one from here:
Humminbird: America
Look towards the bottom of the page under the “Legacy Products”.

With the basic display that these units had it won’t be able to show any detail and so you will have a hard time distinguishing between fish, especially Crappie, and anything else. About all you will be able to go by would be the shape of the school (somewhat Christmas Tree shaped) if you are over schooling fish.