To measure the NC value of noise in your control room or studio, or to verify that a specified NC value is actually met, here's what you do.
You will need a sound level meter capable of reading noise in octave bands. The sound level meter should also come with a measurement mic that has flat frequency and phase response over the entire audio band and low noise. If this is for occasional use, you can rent the instrument.
Many software or software/hardware audio and acoustical measurement tools also provide noise measurements and plots of NC curves (so you don't have to do it manually). If relying on a sound card for the audio interface, you'll need to make sure it has flat frequency and phase response over the entire audio band, just as the measurement mic. Also its noise floor must be lower than the noise level you are trying to measure.
What you want to measure is the background noise of the room with all air handling and lighting turned on at the level most likely to be used (dimmers are good noise sources), and doors closed. If there's equipment already installed in the room, it's a good idea to take measurements first with all of the equipment turned off, then turned on. (You can then easily see the effect of any equipment fan noise.)
Let's say you are using a sound level meter. Set the octave filter settings to the first octave band you want to measure. In this example, let's start at 63 Hz. Stand in the middle of the room, take the noise reading, and note its value. Continue with measurements in each of the other octave bands. Then tabulate the measurements in a spreadsheet, or plot the readings on a graph of NC curves. (A Web search will turn up NC curve graphs.)
The following table shows three NC curves and some sample noise measurements to illustrate how to determine the NC value.
Looking at 8000 Hz, it looks like we hit NC-30. But at 4000 Hz, the "measured" noise level is 1 dB above that for NC-30. So right away, we know we're higher than NC-30, even though the "measurements" for 500, 1000, and 2000 Hz fall below the NC-30 curve.
At the 250 Hz octave band, we have a reading that falls on the NC-35 curve. But at 125 Hz, we're above the NC-35 curve, but below NC-40.
And finally at 63 Hz, we're almost at the NC-40 curve. So, for all practical purposes, this room would have a NC-40 rating. (You could argue that it's NC-39.) At any rate, if the specifications for this imaginary room were NC-30 or NC-35, it doesn't make it, mostly because of higher-than-allowed low-frequency noise for those curves.