If you type arp -a in the terminal/powershell window it should show devices connected to it. The Raspberry Pi will have a MAC address that starts with one of these: https://udger.com/resources/mac-address-vendor-detail?name=raspberry_pi_foundation
In my case my umbrel wasn’t on that list so I had to use wireshark to see it, but it only gives me an IPv6 address… which does not go to the device when I type it in the browser.