Battery is the part of a laptop that wears with use, so it is the first thing to check on any pre-owned MacBook. The good news: macOS tells you everything you need in a couple of clicks.
The quick way
Hold Option and click the battery icon in the menu bar, or open System Settings → Battery → Battery Health. You will see a status such as "Normal" and, on newer versions, a maximum-capacity percentage.
Check the cycle count
Open Apple menu → About This Mac → More Info → System Report → Power and look for "Cycle Count". Apple rates most modern MacBook batteries for 1000 cycles. A unit with a few hundred cycles and 85%+ capacity has plenty of life left.
What we do at Mobirapid
We test and report battery health on every device, and back it with a 6-month warranty — so you never have to guess.