Updates usually come in the form of notifications when they are sent by your phone company. Of course, if your smartphone does not have the ability to update then you will never be notified. You can also go to:
2. About the device
3. Software update
And there you can check with more security if there are any pending updates for your smartphone.
You can also check on the official page of your phone brand, there they always give details and you can even download the update file.