Testing the noise floor is best done with a good quality soundcard and a 'real time analyzer' application.
For a basic noise floor test of a mic input, you terminate the mic-in xlr with some reasonable value resistor or even short circuit the hot-cold phases together. Perhaps someone more expert in this can elaborate.
You then connect the output of the preamp to the soundcard input - good soundcards have balanced ins, outs.
It's very much worthwhile to get a good soundcard with balanced I/O if you are a serious DIYer
The 'RTA' app then shows you the spectrum (graph of amplitude vs freq) of the output of the preamp in 'real time'.
You can vary the gain of the preamp to see the effect on the noise floor.
RTA apps often give you a 'headline' figure for the incoming signal 'energy level' - often referred to as an 'SPL' (sound pressure level). This is the aggregated energy level over the audio bandwidth ie. 20Hz-20KHz or even up to 44Khz or 48KHz.
The SPL can be relative to the soundcard's 'full scale deflection' or FSD which is the max input amplitude before clipping, usually expressed in 'dB' or 'dB below FSD' or it can be referenced to some absolute voltage or power ie. 'dBu or dBV and so on.
The next thing to do is to use the soundcard's output to send a tone (eg 1Khz) of a certain amplitude (eg -10dB or -10dBu) to the preamp and return the preamp;s output to the soundcard. The RTA application then shows the spectrum around and including that test tone - you can figure the gain and distortion of the preamp by measuring the spectrum 'peaks' and so on.
Good RTAs also let you 'send' a frequency sweep to the preamps input and record the preamps output and display a 'freq response' graph at various signal levels.
A good RTA to get is 'TrueRTA'. For the committed diy person on a budget, it's the best. It will grow with you.
edit - it's a PC thing. For apple folk there's probably something on an iphone or whatever
Another one people use is 'Rightmark' - free and good for somethings. But not as real-time usable or awesome as TrueRTA.
PS - what a fabulous build - anyone would be proud to offer that beauty up!
Cheers