First, make sure checking your Operating System (OS) Software Updates is what you are trying to do. Your OS is for your entire device. If this is already up to date or you have already checked it, you might mean to be checking on your Browser Version to see if it needs updates. Your Browser Version is which version of Chrome or Safari you are using and it is sometimes updated separately from your OS Software Update. You can learn how to check for browser updates here.
Mac OS
Go into your settings and click on General on the right side. On the left side, click on Software Update.
If you need to update your software, the update needed will be listed and you can either upgrade it in the moment or schedule it to update overnight.
If you do not need to update your software, this page will only show what software you are running and when it was last checked.
iPad OS
Start in your Settings app
Click on the General tab on the right side. On the left side you will either be shown the software updates that you have available (See Mac OS images) or you will see something similar to the screen below, indicating that your software is up to date.
Windows OS
You can click on "update history" to see which updates have been installed.
Chrome OS
Android OS
Open your settings app and scroll until you find an option that says system or software update.
Your device should automatically check for new updates when you do this. If not, select "check for updates."