How can I ensure that a field has been entered in a Workflow Task before a user can confirm the decision?

From the DocuWare Workflow Designer, from the Workflow Task in question, go to the Decisions Validation section.
Check the box for "Enable data input validation" then use the following Condition, 

!IsNull(Workflow Field) 

Where Workflow Field is the field which is listed in the "Workflow Fields" section in the Conditions window.

The following screenshot shows what it should look like in the workflow designer.

NOTE: This will not work if you choose the field from the "Index Fields" section. 

Alternatively, from the Task Configuration's Dialog section, click on the icon to the right of the field to open its properties. 
From here, you will be able to check the "Required" option to make filling out this field required as well.