Internal sales surveys or qualitative feedback (e.g., 'what decks do you use when pitching?' 'what assets are most helpful?') can work. If you have an internal sales wiki built where you host assets, you may be able to access analytics about how many visits/pageviews/downloads you're seeing across key materials. But most importantly, having strong relationships with sellers and sales leaders can help create a feedback loop for PMM so that the materials that are most needed are the ones being created.
This is tough to measure quantitatively without tooling in place to track how often collateral is used, so if that's an option for you, start there. Then be curious about what assets are used most often because they're familiar/readily available, and what assets are actually the most strategic.
Without tooling, the goal is to create regular (e.g., quarterly) opportunities for your sales team to provide feedback on what they use and why. This doubles as an opportunity to reinforce everything that's available and gather ideas for high-impact new assets!
This is also a great topic for win/loss interviews you're able to schedule. Ask the prospects about the collateral they've seen, what they took away from it, and what (if anything) they shared themselves. It's also a key opportunity to ask what they "learned the hard way" (or were never able to learn at all).
This is always a fun topic!
Here are some approaches I've taken in the past:
One of the KPIs for the PMM team should be around sales enablement. I have seen this KPI measured when a PMM delivers sales training. After every live session, we would do a survey where we check two dimensions: Quality of the presentation and usefulness of the material. Though this was interesting initially, we noticed that there is general fatigue over time, and most people would give a high average rating of over 8 (scale:1-10). There are sales enablement tools like Highspot that provide more visibility into the content impact by "stage," but I've not seen this action.