A Lovely Two device
Lovely Two is a portable love tester popular among girls in the manga.
The owner inputs data including their name, birth date, blood type, which creates a special waveform. By pressing a button, while in range of another Lovely Two, it will check to see if their waveforms match. Matching wave forms will result in a ringing noise, indicating the two owners would go well together romantically.
Anzu and Yugi test their love compatibility twice; The first time they get no reading. The second time they try, Dark Yugi is in control of Yugi's body and a connection is established.