The JNLP file produced by the packager contains a block like this when toolkit = fx:

  <applet-desc width="800" height="600" main-class="com.package.Whatever" name="Whatever" >
    <param name="requiredFXVersion" value="1.8+"/>
  </applet-desc>
  <jfx:javafx-desc width="800" height="600" main-class="com.package.Whatever" name="Whatever" />

If toolkit = swing you get:

  <application-desc main-class="com.package.Whatever"/>

But unless I missed it there's no toolkit value for an FX WebStart application, which requires:

  <jfx:javafx-desc width="800" height="600" main-class="com.package.Whatever" name="Whatever" />
  <application-desc main-class="com.package.Whatever"/>