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Now that depends on what you want. If you are looking for 1-sigma noise voltage, then put a scope on it, turn on trace overlay and see how thick the bar gets for a specific DC input.
If you are looking for noise performance over frequency, hook up a spectrum analyzer and for a given DC input (i.e. set up the chopper as a follower and have it's output mid-rail) look at the spectral power.
If you want an agregate noise to relate to a rms noise voltage or a noise power, you need a power meter. Set the op-amp as a follower, put a power meter on the output but make sure it has a DC blocker on it. The power you read will be your noise power which can be related to a noise voltage.
If you are using the chopper for mid to high frequency signal processing and you want the noise figure of it, then use a Noise Figure module available for most spectrum analyzers. Put the caliberated noise source at the input and measure the noise power at the output.