How can I test how much voltage an electret microphone output?

I want to know how much voltage an electret microphone can output when I scream into it at very close proximity. Can I connect the microphone directly to oscilloscope with BNC to probe wire. With +ve of microphone connected to +ve of BNC to prove and -ve of microphone connected to -ve of BNC to probe.

The microphone should have a sensitivity specified in the data sheet.

No you have to provide power to this sort of microphone:-

Can I connect the microphone directly to oscilloscope with BNC to probe wire.

Yes, but as Mike said, electret mics need power so you need to provide power to the mic per the manufacuter's specs.

And, you might need a resistor (anything around 10K ohms should do) in parallel with the 'scope input. Otherwise you might get a DC offset or DC drift (due to leakage through the capacitor into the very-high scope/probe impedance).

I want to know how much voltage an electret microphone can output when I scream into it at very close proximity. Can I connect the microphone directly to oscilloscope with BNC to probe wire.

I'm going to guess you should be looking for 100mV to 1V.

In the attached photo is the +ve pin of 470nF connected to +Ve pin of electret microphone? And the -ve pin connected to oscilloscope?

The 470nF is not polarised so it doesn't have a +ve pin. Basically yes. But it is an oscilloscope so you can connect it any way you want.

A typical ECM has a sensitivity in the -44 dBV/Pa range (0 Pa = 94 dB SPL).

Normal speech is ~ -5 dBPa in measure 25 mm in front of the mouth. (this is the commonly used reference level, but will in reality vary videly with different users.

Loud yellling will probably give ~ 125 dB SPL (or ~31 dBPa) in the same distance, again varying from person to person.

So, given your bias circuitry and microphone can handle this you should get: -49 dBV (3.5 mV) for normal speech -13 dBV (224 mV) in the loud example.