Open Task Manager and click on Details tab. Scroll down and look for MsMpEng.exe and the Status column will show if it's running.
Defender won't be running if you have another anti-virus installed.
Also, you can open Settings [edit: >Update & security] and choose Windows Defender in the left panel. This will give some settings and at the bottom is 'Use Windows Defender'. This will show the last time there was a scan and if it's more than a day you
can perhaps run a quick scan.