It's not practical to do in Marketo, since if the person never fills out the form completely, you won't know who they are -- and Marketo is not the place to report on sessions while they're still anonymous (it's fine for reporting on previously anonymous activity, of course).
This would be pretty straightforwardly done using GA (or similar anonymous-centric analytics) but it's more common to A/B test your forms using different field counts. (Though I agree that deprives you of a super-deep dive that would be interesting.)
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