Well, just spit balling, but you should see it in team velocity. The effort you're spending coaching should show up in the output of your team members.
Assuming there's some kind of constant ongoing engagement with one person or group, I'd expect an immediate dip in velocity as your productivity goes elsewhere, then it growing, and maybe evening out to before-engagement numbers (approximately), as they reap the benefits of learning from a senior engineer. Then, as you're able to return to normal duties and they're able to apply the lessons, you should see velocity greater than before the engagement. That delta should be somewhat quantifiable and, ignoring other variables, should represent the benefits of coaching.
There's also huge value in the increased job satisfaction for both mentor who enjoys mentoring, and a mentee who is learning. That should show up in any kind of employee satisfaction survey, or retention numbers.
Assuming there's some kind of constant ongoing engagement with one person or group, I'd expect an immediate dip in velocity as your productivity goes elsewhere, then it growing, and maybe evening out to before-engagement numbers (approximately), as they reap the benefits of learning from a senior engineer. Then, as you're able to return to normal duties and they're able to apply the lessons, you should see velocity greater than before the engagement. That delta should be somewhat quantifiable and, ignoring other variables, should represent the benefits of coaching.
There's also huge value in the increased job satisfaction for both mentor who enjoys mentoring, and a mentee who is learning. That should show up in any kind of employee satisfaction survey, or retention numbers.