spectrum analyzer readings of clear frequency

Hi experts.

Does anybody have there a guide or has the knowledge to answer this:

How with an spectrum analyzer can you say that a frequency slice is free of interference or unwanted signals. We want to know if a frequency range is free so we can turn on something on it.
I know about the thermal noise equations that involves the temperature at 300K and the range of frequency, but is that enough to go on with a spectrum analyzer and point to everything above that value as a dirty/used/interfered frequency??

Thanks a lot.

I guess, it depends on the frequency range you are interested in. For example, most transmitters in the 2.4 and 5 GHz bands make a unique shape, or "signature". Since they all look a little bit different, we can use these signatures to identify what kind of device is transmitting, which makes tracking things down a lot easier. See

https://support.metageek.com/hc/en-u...rence-Examples