
Originally Posted by
Dishmaster
In principle yes, but there are so many other heat sources in the universe that it becomes quite difficult to disentangle all the contributions.
You can't use a simple thermometer for this purpose, but you can measure the energy distribution spectrum of the radiation, like it was done with COBE. It is a continuous Planck spectrum of a perfect Black Body. It is uniquely determined by the temperature. The peak of the distribution tells you exactly, what the temperature is.
Disentangling can be done by spatially resolving the different contributors. The image below contains lots of radiation coming from the Milky Way. This must be subtracted from the data by applying models in order the retrieve the background radiation alone.