Check the existence of a field in a Content Object in eZ Publish in Twig
To check the existence of a specific field you can use Twig in eZ Platform:
{% if content.fields.cool_field is defined %} This content has a cool_field field! {% else %} This content does NOT have a cool_field field! {% endif %}
Kudos to Edi Modric for the tip, I was wasting time with this.