In OPAmp datasheets normally the input referred noise is given. You can get the value of it with simulation of the power spectral density function (PSD).
Connect your OPAmp as unity gain buffer, with a DC source sets its common-mode input voltage in the middle of your common-mode range, then run AC noise analysis (.noise as sutapanaki said).
The analysis automatically adds the theoretical AC noise sources to the small signal equvivalent model of your OPAmp.
After the simulation plot the output node's noise vs. frequency curve (you can pick it from the results browser after simulation or write an expression at your signal outputs, like: VF("/opamp_output_node_name") ).
In unity gain buffer connection the output noise is equal with the input reffered noise at the center of your bandwidth, its unit should be nV/sqrt(Hz), sometimes V^2/Hz, they are the same basically. Or as sutapanaki said, bit advanced, but you could define an input source and an output node in the analysis and it can tell you the exact input referred noise over the full bandwidth, even your OPAmp has a limited bandwidth or you are using it in a different connection (like an inverting amplifier with bigger gain than 1 for example).