Managing outside salespeople is akin to managing the invisible. Unlike inside salespeople and other operational staff members who work on site, Outside salespeople work independently, unsupervised, and out of sight for the majority of their work week. This leaves managers wondering how their salespeople prospect, present, and manage conflict in the field while feeling powerless to affect sales performance.

The sole measurement of success becomes sales results, but even this produces doubt for managers. One told me he feels like the top salespeople in his organization are “the last men standing. They inherited the accounts of the ones who quit and I don’t even know if they are really producing results.” This manager is not alone.

Many complain about their salespeople while failing to spend time in the field with their salespeople to fix the problem. But there is something managers can do: It’s the lost art of coaching. Here are the methods that I encourage my clients adopt:

Manage One Individual at a Time. The most powerful lesson you can learn as a sales manager is that group meetings and sales training are no substitutes for one-on-one coaching in the field. Each salesperson has different skill levels to be addressed. It’s a slow process to coach each individual on your team, but the results are lasting. Most important, it’s the only way you can validate the performance you expect.

Don’t Surprise Your Salesperson. There is debate on this subject. Some managers believe the best way to observe the invisible is to surprise salespeople by scheduling coaching time at the last moment. I strongly advocate scheduling time in the field in advance. Your goal is to see what your salesperson is capable of under the best possible circumstances.

I prepare salespeople for coaching sessions by telling them to “plan the perfect day.” This means a series of scheduled meetings with a mix of customers and prospects. An ideal coaching day includes three or more appointments and a number of other planned stops during a full day on the road. If your salespeople can’t prepare quality days of activity when given fair warning, you can’t have any confidence they’ll do it when you’re not looking.

Focus First on Science of Selling. The biggest coaching mistake I see is when sales managers start debating the skills I refer to as the “art” of selling. This includes presentations, conflict resolution, and closing. It’s easy to give feedback on these skills, but often useless because there are no definitive answers. On the other hand, the “science” of selling includes such quantifiable information as prospecting data, closing ratios, and number of sales calls. Manage the science of selling to remove all ambiguity and debates about performance.

Know When to Talk. During your joint calls, aim to reveal how the salesperson behaves when working independently–i.e. while they’re invisible to you. If you are meeting with a key customer, the manager probably needs to be involved and speak up. But on other sales calls, it’s best to let the salesperson handle the meeting so you can observe the performance you can expect when you’re not present.

You’ve probably heard it said that you should inspect what you expect. The goal of a quality coaching session is to reveal the performance a manager can expect when the salesperson is out of sight. It’s also the best way to clarify performance expectations and shape the sales behaviors that work.