rtl_power frequency hops at about 80 MHz/s max. You can use https://github.com/xmikos/qspectrumanalyzer or many other rtl_power frontends to see it in real time as you point the antenna. For windows there's https://github.com/pavels/spektrum

Another approach seen on hackernews in the past had an omnidirectional antenna and used GPS tagging to make heatplots at different frequencies overlayed on a map. ref: http://www.tautology2.net/heatmap/ It uses the http://eartoearoak.com/software/rtlsdr-scanner software.