First, click the Apple logo in the upper left-hand corner.
Click "System Preferences."
In the left-hand column, select "Battery."
Here you can see your battery health, which should ideally say "Normal."
You can also see the maximum capacity of your battery, which tells you if your battery can charge to the full 100% capacity. Naturally, batteries degrade overtime, so as long as your battery is above 75%, you should not consider replacing your battery unless it has exceeded the maximum cycle count.
However, here, instead of "Normal," it may say that your Mac needs service. If it does, this means that you simply need to look into servicing your battery at Apple or an Authorized Service Provider.