You can find out without lifting, moving, or unplugging anything. Just Option-click the battery icon in the Menu Bar. A drop down menu will appear that tells you the status of your battery. You will see Normal, Replace Soon, Replace Now, or Service Battery, right next to the word Condition.
Click that entry and an Apple Help page will open, explaining what each term means if you need more information. Apple also has a support document that explains more about battery condition.