How to test which CTAs prompt users to take specific actions

I'm experimenting with different messages/Calls-to-Action in our product to see which specific CTAs are most successful at prompting our users to take specific actions we want them to take.

Besides testing different CTAs quantitatively (through A/B testing, or by tracking email open rates & task completion rates—where each email has a different CTA), does anyone have experience testing this sort of thing qualitatively?

In usability testing, I'm going to ask people to paraphrase the CTA in their own words. But in terms of testing whether a CTA prompts users to take certain actions, have any of you tested that qualitatively?