How to check a laptop's battery health

Windows

1. Open Command Prompt.
2. Type powercfg /batteryreport and press Enter.
3. Open the HTML file it creates (the path is shown).
4. Compare Design Capacity with Full Charge Capacity.

If full charge capacity is 40,000 mWh and design capacity is 50,000 mWh, the battery is at 80% health.

Mac

1. Hold Option and click the Apple menu.
2. Choose System Information → Power.
3. Check Cycle Count and Condition.

On recent macOS versions, System Settings → Battery also shows a maximum capacity percentage.

Lenovo Vantage, Dell Power Manager and HP Support Assistant

These manufacturer apps also show battery health.