You will not be notified. You should see a version number near the MODX icon in the top left hand corner of the manager. (Although I think this was omitted in one of the older versions).
If you install the UpgradeMODX 1.5.5-pl extra you will see a widget in the manager dashboard that will display your current version number and inform you of the latest version number and the need to upgrade. This extra, by Bob Ray, makes the whole process of upgrading much easier than it used to be.
https://modx.com/extras/package/upgrademodx
You should also upgrade any extras you have installed. Do this through the Extras / Installer menu.
Please ensure that you have a backup of both your database and your web site files before you do any of the above.
Once you have done all the above you might also consider improving the security of your site by renaming and moving the 'core' folder and renaming the 'connectors' and 'manager' folders as described in this document:
https://docs.modx.com/revolution/2.x/administering-your-site/security/hardening-modx-revolution