Regular reminders and reinforcement! Sales teams obviously face a ton of pressure to hit quota, and they need to be as efficient as possible... And while some are awesome at experimentation, many will struggle to adopt new assets/messaging when they're moving fast and rely on what's familiar.
On the flip side, be sure to manage outdated materials as best you can... If there are old decks/n-pagers floating around, label them as such, and if you see things pop up from people's "private collections" send them a note with the latest and greatest and ask them to replace.
Make it as easy as possible to use a new asset (discoverable, clear guidance) and then showcase where it has been impactful for other reps. Nothing like an internal case study to motivate your team!
First, I think you shouldn't expect 100% wholesale usage of the sales enablement materials you create. Sales enablement materials are meant to create a solid foundation for reps to use but, ultimately, every prospect and customer requires some level of customization so expect adaptation.
There's really a spectrum here of tracking and it depends on what metrics you need to justify the resourcing you put to sales enablement, and to whom you need to justify that resourcing. On one hand, you can simply use anecdotal feedback by talking to reps and sales leadership to understand if the materials are being used. This could come from joining sales meetings and asking, or doing an informal poll. On the other end of the spectrum, you could gate and tag every marketing asset and use technology solutions to track usage. I'm not going to cite specific tools but it won't take long for you to find!
The more tracking you do, the more friction you might introduce into the process and in some ways it might actually run counter to your goal (usage). But if your team needs to justify additional heads for sales enablement, or you need more OpEx to get things done, you could take this approach.
First, create content that’s in line with what you know they will use. I mentioned in one of my other answers here that oy need to understand their selling methodology and ensure the content aligns with it. If they use Demo2Win then know your content needs to follow a Tell-Show-Tell framework around problems, solutions, value for each mini-chapter. If they use ValueSelling, know that your content needs to anchor on a key business issue followed by the common obstacles standing in the way. The more you understand about how their pitch will be run, the better you can build content that will easily fit into it and get used.Second, ensure everyone actually knows the content and is comfortable with it. Rolling out decks and hoping they get used can be frustrating for everyone. Add speaker notes to the slides, record talk-tracks to the decks that people can review, and run elevator pitch competitions where each person needs to present an overview of the content to their peers and you. Offer up some prizes to make these fun and incentivized.