How do you find a lost serial number for a stolen Mac to make a police report?

If it’s signed into iCloud, you can see it and its details under “My Devices” in your Apple ID. See here for instructions from Apple.

You do easily determine the serial number of all the devices linked with your Apple ID using just the Apple ID, without needing to have access to the product box or receipt.

Log into Manage your Apple ID website with your Apple ID using a desktop Web browser.

Once logged in, scroll down to the Devices section, locate your device and you can find the Serial number.

Alternatively, if you have an iPhone/iPad logged in with the same Apple ID, it's much easier to determine the Serial number for all the devices linked to your Apple ID.

On your iPhone/iPad, go to the Settings app → Tap on your name shown at the top. On the next screen, scroll down until you see the list of all the devices linked to your Apple ID and tap on the device name.

The next screen would show the serial number of your Mac just like how it was shown on the website.

You can take either one of the approach that's the most convenient for you.