A really good way to find fissures is to pop a prospector limpet into a promising looking asteroid and all of the fissures (if any) will be marked. Then you can examine them up close to familiarize yourself with what to look for. I never worry about it. I have simply learned to recognize mother-lode asteroids by appearance, then pulse wave analysis, then confirm using a prospector limpet. A prospector limpet also marks all the fissures for optimal seismic charge placement.