What's the best way to rapidly test if your messaging is working as intended? Related question - how do you know if your messaging is working or not?

The best way to rapidly test whether your messaging is working as intended is to deliver it to your sales reps or other collaborators and see whether it helps them or not. Ideallly, you'd equip a few of them with a beta of your new pitch deck, or messaging artifacts and either get their gut reaction right then and there, or have them pilot it on a few conversations.

Of course, if we're talking about digital channels than quick a/b tests via tools like Optimizely on the web, or tweaks you make to your assets like newsletters and nurtures that you test will also give you a sense of how your messaging performed. 

At Matterport we recently refactored our product page for Matterport for the iPhone. We had a strong start when we launched the page at the time of the product announcement, but then traffic slowly started dwindling there. When we refactored it we found that the primary driver for people coming back and staying on the page was the addition of customer use cases. Another example of where customer stories are literally the best messaging you can come up with. 

The best way to tell that your messaging didn't resonate is that reps end up going back to their own content or ignore the deck and assets you created. I've often seen companies where sales just putting their own messaging together because product marketing didn't meet their needs. See my answer above on how to keep sales in the loop during messaging development. 

Getting messaging into the market is the best true test. Depending on the key channel you wish you utilize the messaging this may come in the form of marketing or sales initiatives and varies based on the investment. You could start with a landing page to A/B test conversions or with a little budget a webinar covering the new messaging, promoted by digital ads, with open Q&A and a feedback survey. Alternately you get gut reactions from a few sales executives, and request they use it in an upcoming conversation with target audience, then review Gong recordings of the response/success in the wild to gauge how it landed. 

For rapid message testing, I'll rely on channel metrics that we've set baselines for in the past. Ideally you're able to identify message tests as experiments with explicit goals, for example, to increase demo request conversion.

Cross-posting an answer to your related question:

To measure market messaging, I focus on resonance, clarity, and differentiation. Whenever possible, I use a mix of qual and quant strategies to get data, but in my experience, even a handful of qualitative interviews with target personas provides a treasure trove of insight. What matters most is getting outside the metaphorical building to really pressure test what can often be strongly-held internal opinions.

Measuring sales messaging is also best done as a mix of qual and quant. I'm fortunate to work with a team of thoughtful, strategic sellers who are generous in bringing back feedback from the field. We'll often trade insights from mini-experiments over slack or in team meetings, or, when we have a larger rollout (like a new pitch deck), we build in dedicated feedback sessions. I also carve out time to listen to gong calls to see directly a) how messaging is delivered and b) how it's received. The questions that prospects ask and where they lean in vs. tune out speak volumes in terms of what's resonating, what's clear, and whether it feels differentiated.

These qual efforts help provide real-time insight. Ultimately, of course, we measure the impact new messaging has on sales stage progression. We have longer, enterprise sales cycles, so this takes time, and of course there are always multiple factors playing into win rate. That said, it's the most important metric we're all looking to impact, so it has to be central.

The simplest here is if you're moving awareness, pipeline, close rates, revenue, etc. But I know that's not always immediately available. So I have a simpler answer. PITCH YOUR MESSAGING TO PEOPLE, IN PERSON. This might seem too basic but I've seen the success over and over. Pitch your messaging to customers, key stakeholders, your sales team, anyone that will listen. It's really easy to write bad messaging when you never see a human face react to it. Pitch it, iterate, pitch it, iterate, keep going! And messaging is never "done", embracing this concept is very liberating as a PMM;)