Where is the Disk Utility log? (Mac OS X)

It's in ~/Library/Logs/DiskUtility.log

For anyone on Mojave, it seems there's no longer a .log file to tail.

Instead, open up Console.app and click on your computer name under Devices in the left sidebar.

Then, you can search for "disk utility" in the top search bar. After pressing enter, click on the drop down next to Any (in the search bar) and click Process. That'll show you active log items from the Disk Utility process.

For me, a drive First Aid was in progress and the Disk Utility window was beachballing. This allowed me to see that Disk Utility was actually still doing work (and I should be patient).