Look at the battery icon in the menu bar (top-right) to confirm it says “Charging” or shows a charging indicator
If you use System Settings, open System Settings → Battery (or Power Adapter) and check whether it indicates “Charging”
Check whether the power adapter is connected firmly to both the Mac and the wall outlet
Verify the MagSafe/charging cable connection is secure (when connected, you should see the charging indicator behavior you normally get)
Look for a change in the battery icon state from “Not charging” to “Charging” after plugging in
If you have a MacBook with an LED on the adapter, confirm the LED is lit when plugged in
If the battery is full, the Mac may show “Not charging” while still being plugged in (or it may stop charging)
Check for any notification or status message near the battery icon indicating power/charging status