Testing an Android screen in the browser
Open this page in Chrome or another Android browser and tap Start multi-touch test. The test area goes full screen, and Chrome’s pull-to-refresh, pinch zoom and scrolling are off inside it, so a downward swipe draws a trail instead of reloading the page.
Android’s own feature list sorts touch hardware into levels that apps can ask for. Basic multitouch covers two-point gestures such as pinch without following each touch separately. Distinct multitouch follows two or more points independently, and the level Android calls jazzhand follows five or more. If this test never gets past two, the phone may only have the first level, or the screen is losing fingers.
Then switch to Dead zones and cover the grid, edges included. Android uses edge swipes for Back, Home and the notification shade, and it can claim a stroke before Chrome passes it on, so begin each edge stroke a centimetre or so in from the border.
Compare with Android’s own tools
Android’s Developer options have two touch displays that work outside the browser. Show taps draws a circle under each finger, and Pointer location draws crosshairs and the touch path with a bar of coordinates along the top. To turn on Developer options, tap Build number seven times (on a Pixel it’s in Settings > About phone; the place varies by device). If a dead zone or ghost touch shows up there too, it isn’t the browser.
For phantom taps, leave the phone flat on a table and run the Ghost touch watch, first on battery and then plugged in.
Sources: Android Developers: uses-feature, Android Developers: Configure on-device developer options.