I am constantly testing messaging. Any customer meeting, event or executive briefing I attend, I'm testing messaging in some way, even if it's not messaging that I'm actively working on. The most critical part of this process is to ensure you're leaving space for feedback and reactions or explicitly asking for it. One mistake I used to make when I first started as a PMM was to go into presentation mode and just barrel through a deck/pitch instead of adding pauses and deliberate questions throughout. As a result, when I build narratives, I think about questions I should be asking about and will bake those into speaker notes. I've also added random dots in presentations that only I can see as a visual cue to take those pauses. We oftentimes find silence is uncomfortable, but it's actually a friend of PMM because it encourages customers to fill that silence.
In addition, I also recommend testing messaging directly with your field and customers and looking for methods to test outside of your organization's typical customers/prospects. We will oftentimes do the latter using user research or advertising.