# How do we tell the CMB apart from other radiation?

Say I want to observe the CMB and the CMB only.

I point my device (telescope in some frequency range) at the sky and start looking. How do I know it should be in the Microwave spectrum? How do I distinguish it from other radiation?

To expand on this, you would point your telescope at an empty patch of sky (as mentioned). You expect the CMB to be thermal to a very high degree ($\sim 1$ part in $10^5$). Also, you can point your telescope to any direction on the sky and get (statistically) the same observation (if you have the resolution to pick up the primordial fluctuations that are $\sim 1$ part in $10^5$, despite those two regions begin causally disconnected (they lie outside each others' horizon, e.g. they never communicated).