How to Test a Used Phone or Monitor Screen Before Buying

The screen is the most expensive single component in most devices and the one whose faults are easiest to hide in a dim room or behind a busy wallpaper. It’s also the component you can inspect completely, in about ten minutes, with nothing installed — which is the entire reason our tests run in a browser. Open the site on the actual device, in front of the seller, and let the panel testify.

Before you leave home

Know what a replacement screen costs for the exact model you’re buying. A cracked-glass phone might be a $90 fix; a phone whose OLED needs replacing might cost more than the asking price. That number turns everything you find below into a negotiating position instead of a vague feeling.

The routine

1. Solid colors first (3 minutes)

Open the dead pixel test and step through all nine colors, looking over the entire panel each time.

You’re looking for more than dead pixels:

• Dots — dead (dark on white), stuck (colored on black), hot (white on black). One stuck subpixel is a bargaining chip; a cluster is a walk-away.
• On black, in a dim setting: backlight bleed and clouding on LCDs — glow leaking from edges or blotchy bright patches. Every LCD has some; you’re judging whether it would annoy you in a dark room.
• On white: tint shifts and mura — is the panel evenly white, or pink in one corner and green in another?
• On gray (the most revealing single screen): OLED burn-in. Ghosts of a keyboard, a status bar, or a channel logo mean the previous owner left static content up for months. Burn-in only ever gets worse.
• On red, green, blue individually: on OLED phones, uneven wear shows per-channel; a faint shadow visible only on blue is early burn-in that gray hasn’t caught yet.

2. Gradients (2 minutes)

Switch to the gradient test on the homepage and run the grayscale ramp both directions. A healthy panel shows a smooth sweep; coarse stair-stepping can mean the panel or its cable is running at lower bit depth than advertised, and one-sided discoloration you didn’t see on solids often shows here. (What banding is and isn’t — some banding is normal on 6-bit and 8-bit panels.)

3. Touch and digitizer, phones only (2 minutes)

A screen can look perfect and be unusable. With any drawing app — or just the home screen edit mode — drag an icon slowly across the full screen, all four edges. Watch for places it stutters or drops. Dead digitizer zones cluster near past repairs and are invisible until you own them.

4. Brightness extremes (1 minute)

Max brightness on a white screen: does it actually get bright, or has someone’s aging backlight been compensating? Minimum brightness on black in your cupped hands: flickering or unstable dimming at the low end is a common fault on repaired panels.

5. The repair question (2 minutes)

Angle the screen against the light and look at the edges. Original panels sit flush with even gaps; aftermarket replacements often show lifted corners, uneven glue lines, or a slightly wrong glass tint. On a phone, ask directly whether the screen has been replaced — an original-parts replacement done well is fine, a $30 aftermarket OLED is not, and a seller’s hesitation is data.

Reading the results

One honest caveat

A ten-minute test catches the faults that exist today. It can’t see a flaky cable that fails when the laptop lid flexes, or a panel that misbehaves only when warm. If the deal allows it, keep the device running through your whole meeting — intermittent faults love the first thirty minutes.