Plug in, observe charging stability, and watch for unusual heat. Check cycle count where supported and confirm maximum capacity percentages. Test fast‑charge with a compatible brick to rule out port issues. Let the device idle, then stream video to compare thermals. A healthy battery shapes daily experience far more than tiny cosmetic scuffs. If numbers look marginal, negotiate, ask for a replacement, or walk away before small annoyances grow expensive.
Run a full‑screen white and black test to spot dead pixels or uneven backlight. Check touch responsiveness at edges, then open the camera, switch lenses, and shoot low‑light photos to expose focus quirks. Wave a hand for proximity, rotate to test gyros, and try portrait or night modes. These tests take minutes yet mirror real life. A brilliant camera and clean screen frequently justify small cosmetic wear that saves significant cash.