Thanks for the suggestions, I can try audios method, but Nigels suggestion does make sense, and that is how I've thought of doing it. What has made it impossible is that the application is a microphone, and its very difficult to keep everything quiet around it whilst testing. This is the circuit:
"Better electret microphone phantom powering circuit"
https://www.epanorama.net/circuits/microphone_powering.html
Is there a way to test this for noise without the microphone capsule in place? The capsule uses 0.5mA and has around 10v across it, would placing a 20k resistor where the capsule is shown work to test it? The reason for this is i'm sure i'm getting more noise than I should be (I used BC177s in my current mic but am now realising they arn't that quiet compared to others) and with 2 capsules still left (these are the nice panasonic WM-61As, I finally found them in the UK) I want the next ones to be as low noise as possible.