Question: How do I test the amount of noise added by a mic preamplifier?

Answer: First you need to terminate the input of the mic pre by connecting pins 2 and 3 together.  This can be done with a paperclip inserted into pins 2 and 3 on the microphone input.  If you have a microphone plugged into the mic input, then you are hearing the self-noise of the mic AND the noise of the preamp.  If you have it connected to nothing then you are hearing an un-terminated connection which is likely to include electro-magnetic and radio interference.

With pins 2 and 3 shorted, you can easily evaluate the noise added by the pre-amplifier either by ear or with your favorite meter.