Monitor bookings growth and sales team productivity. Also, jump on tons of sales calls (you should be doing this anyway). Are they on message? Are they handling objections you taught them to handle? It should be obvious to you very quickly whether they are using your stuff.
You have several products with release dates next to each other and limited resources, so what do you do? Here’s how you can think of this: first, identify the releases with the highest ‘tier’ or ‘priority’ (classification of release tiers vary company by company). The highest priority feature is typically the one with the highest impact in the market and that should get more enablement focus.
If you want any kind of quantitative data, purchasing a tool is really the only way to achieve that. I recently evaluated all the sales enablment options and went with DocSend. You can track any content engagement to accounts in SalesForce which you can tie to eventual Closed Won and Closed Lost to track true ROI.
I agree with the above comments for qualitative evaluation. It's all about building good relationships with sales so they will be transparent and candid about what works and doesn't work...with useful feedback.
There are a lot of messaging frameworks out there to choose from, but I take a bottom up approach: I start with the differentiators and proof points and then build my elevator pitch, value prop statements and long descriptions from those foundational components. I also use the rule of 3 for my differentiators and proof points. If you find yourself with a laundry list of differentiators or proof points, start looking for similiarities among those components to create larger "buckets" so that your audience has an easier time remembering your message.
These are all interrelated.
Messaging: Includes value propositions, your story, and pitch. Also includes things like naming, alternatives, and taglines.
Value Proposition: These are the top benefits you want to focus on for your product based on customer and competitive unput
Pitch & Story: These should be the same. Your pitch about the world before your product, the current approach, why it’s bad, the business consequences, and the new world with your product should tell a story. This story should hit on your main messaging points and value propositions.
Hope that helps!