You can check if it's required by going into your Field Configurations settings page under JIRA's Administration panel, then clicking on your scheme. You'll see a
REQUIRED flag right next to the field's name, like shown below.
If you are interested in making this test while the ticket was already created, you can add the following script to your workflow Validator.
cfValues['Your Field Name']!=null
If you are looking to make a field mandatory from ticket creation - follow Joao's answer.
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