This is easy to do:
1) Click on "buildslaves" at the top: https://uberchromegw.corp.google.com/i/chromeos/buildslaves
2) Find the slave.
3) If it has more than one builder in its builders list, it is a floating buildslave: https://screenshot.googleplex.com/M21jfUgtPHL

To find out when a buildslave is busy, click on its link. If it has builds listed under "Currently Building", it's busy: https://screenshot.googleplex.com/P5sjL9AQKjF

Yes, that's what I just did, but it requires some knowledge of how the slaves are allocated (I added a YAQS entry but most gardeners are not going to remember that), and they are only going to do that when investigating a problem. This could potentially give us a heads up in advance, or assist in identifying the problem sooner (without pestering the infra team).

Consider this a suggestion, feel free to ignore.

This is specialized for CrOS and would require some BuildBot code plumbing to enable. Since the current UI is sufficient and this sort of thing should be automated anyway, I don't think investing in augmenting the UI with this information is very valuable.

Marking as fixit to automate the flow.

This issue has been Available for over a year. If it's no longer important or seems unlikely to be fixed, please consider closing it out. If it is important, please re-triage the issue.

Sorry for the inconvenience if the bug really should have been left as Available. If you change it back, also remove the "Hotlist-Recharge-Cold" label.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

This will be completely resolved by Swarming as we'll no longer be tethered to a single machine. This failure mode won't happen. Timeline: 2-3 months from now, at the latest.