The most comprehensive way to measure the effectiveness of your sales content would be to implement a content management tool. Such a tool serves the materials to your reps, produces reporting, and allows you to connect the content usage to your outcomes. Pretty cool. I've been considering a tool like this but the cost and installation effort are still overkill for my company with 1000+ employees, so you probably want more of a "hack". (I can't recommend a particular tool yet as I haven't implemented one).
So absent a tool I try to interview reps periodically to find out what they are using most. Feedback isn't data but works because it is usually consistent from rep to rep. For example I learned that our reps use 2 of my sales slides far more than anything else---great to know!
I also try to build relationships with the reps so they will tell me what they need, if they can't find it. I keep track of these requests. For example, I got a series of comments that suggested my product messaging was too "high level" for our prospects, I focused new content on more tactical use-case messages and the reps feel it works better.
I also look at engagement measures that I can prove such as downloads, qualified leads, etc. This shows what top-of-funnel content I should add to or maintain more frequently.
Finally, I try to get a periodic analysis that connects the content used to the deals we win. In an org my size I try to get a sales ops analyst to do this for me.
So, in summary, try this periodically (maybe once a quarter):
1. Interview good reps about what they use.
2. Review engagement metrics available to you
3. Review deal wins and connect to the content used
The best content should reveal itself immediately!