This is the way to check this.  I've just coded this and submitted it for review with
supporting changes.

@@ -1969,6 +1975,32 @@ check_zoneconf(const cfg_obj_t *zconfig, const cfg_obj_t 
*voptions,
                result = ISC_R_FAILURE;
 
        /*
+        * Check that a RFC 1918 / ULA reverse zone is not forward first
+        * unless explictly configured to be so.
+        */
+       if (ztype == FORWARDZONE && (rfc1918 || ula)) {
+               obj = NULL;
+               (void)cfg_map_get(zoptions, "forward", &obj);
+               if (obj == NULL) {
+                       /*
+                        * Forward mode not explicity configured.
+                        */
+                       if (voptions != NULL)
+                               cfg_map_get(voptions, "forward", &obj);
+                       if (obj == NULL && goptions != NULL)
+                               cfg_map_get(goptions, "forward", &obj);
+                       if (obj == NULL ||
+                           strcasecmp(cfg_obj_asstring(obj), "first") == 0)
+                               cfg_obj_log(zconfig, logctx, ISC_LOG_WARNING,
+                                           "inherited 'forward first;' for "
+                                           "%s zone '%s' - did you want "
+                                           "'forward only;'?",
+                                           rfc1918 ? "rfc1918" : "ula",
+                                           znamestr);
+               }
+       }
+
+       /*
         * Check validity of static stub server addresses.
         */
        obj = NULL;