diff --git a/returnn/tf/util/data.py b/returnn/tf/util/data.py
index 5dec156e1..1ef76aa24 100644
--- a/returnn/tf/util/data.py
+++ b/returnn/tf/util/data.py
@@ -792,6 +792,59 @@ def vocab(self, vocab):
     self.get_same_base()._vocab = vocab
 
 
+# Global dim tag placeholders.
+BatchDim = DimensionTag(kind=DimensionTag.Types.Batch, description="global batch")
+
+
+class _ImplicitDim:
+  """
+  Represents an implicit dim (dim tag) in :class:`Data`.
+  https://github.com/rwth-i6/returnn/issues/706
+  """
+  def __init__(self, tag):
+    """
+    :param DimensionTag tag:
+    """
+    self.tag = tag
+
+  def __repr__(self):
+    return "%s(%r)" % (self.__class__.__name__, self.tag)
+
+  def _eq_tuple(self):
+    return self.__class__, self.tag
+
+  def __hash__(self):
+    return hash(self._eq_tuple())
+
+  def __eq__(self, other):
+    if isinstance(other, _ImplicitDim):
+      return self._eq_tuple() == other._eq_tuple()
+    return False
+
+  def __ne__(self, other):
+    return not (self == other)
+
+
+class ImplicitSparseDim(_ImplicitDim):
+  """
+  Represents an implicit dim via Data.sparse_dim.
+  """
+
+
+class ImplicitDynSizeDim(_ImplicitDim):
+  """
+  Represents an implicit dim via dynamic dim sizes.
+  https://github.com/rwth-i6/returnn/issues/706
+  (For example via :class:`CumConcatLayer`.)
+  """
+
+
+class VerifyOutShapeException(Exception):
+  """
+  Exception via :func:`Data.verify_out_shape`.
+  """
+
+
 class BatchInfo:
   """
   A batched tensor is a tensor with batch dimension,
@@ -1793,6 +1846,51 @@ def get_runtime_sanity_check_op(self):
           checks += [dyn_size_ext.get_runtime_sanity_check_op()]
     return tf.group(*checks)
 
+  def verify_out_shape(self, out_shape):
+    """
+    Verifies that ``out_shape`` matches our shape, i.e. specifically the dim tags.
+      https://github.com/rwth-i6/returnn/issues/706
+    Throws an exception if this is not the case.
+
+    :param set[DimensionTag|_ImplicitDim]|tuple|list out_shape:
+      It must be a set, with the only exception when it is empty (then it doesn't matter).
+      See :func:`dim_tags_set`.
+    """
+    self_dim_tags = self.dim_tags_set_implicit
+    self_dim_tags_implicit_only = self.dim_tags_set_implicit_only_wrapped
+    if not out_shape:
+      if self_dim_tags:
+        raise VerifyOutShapeException(
+          "%s verify_out_shape, with dims %s, does not match empty out_shape %r" % (self, self_dim_tags, out_shape))
+      return
+    if not isinstance(out_shape, set):
+      raise TypeError("%s verify_out_shape: expects a set but got %s" % (self, type(out_shape)))
+    remaining = set(self_dim_tags)
+    for dim in out_shape:
+      if isinstance(dim, DimensionTag):
+        dim_tag = dim
+      elif isinstance(dim, _ImplicitDim):
+        dim_tag = dim.tag
+        if dim not in self_dim_tags_implicit_only:
+          raise VerifyOutShapeException(
+            "%s verify_out_shape, with dims %s, with out_shape %s, %s is not an implicit dim in self" % (
+              self, self_dim_tags, out_shape, dim))
+      else:
+        raise TypeError("%s verify_out_shape with out_shape %s: expect dim tags but got %s" % (
+          self, out_shape, type(dim)))
+      if dim_tag not in remaining:
+        if dim_tag in self_dim_tags:  # can happen e.g. if specified once as implicit dim and then also as explicit
+          raise VerifyOutShapeException(
+            "%s verify_out_shape, with dims %s, does not match out_shape %r, dim %s multiple times in out_shape" % (
+              self, self_dim_tags, out_shape, dim))
+        raise VerifyOutShapeException(
+          "%s verify_out_shape, with dims %s, does not match out_shape %r, %s not in self" % (
+            self, self_dim_tags, out_shape, dim))
+      remaining.discard(dim_tag)
+    if remaining:
+      raise VerifyOutShapeException(
+        "%s verify_out_shape, dims %s are not specified in out_shape %s" % (self, remaining, out_shape))
+
   def get_placeholder_kwargs(self, with_batch=True):
     """
     :param bool with_batch:
@@ -2860,6 +2958,53 @@ def dim_tags_sparse(self):
       return self.dim_tags
     return self.dim_tags[:self.feature_dim_axis] + self.dim_tags[self.feature_dim_axis + 1:]
 
+  @property
+  def dim_tags_set_implicit_only_wrapped(self):
+    """
+    :return: Dim tags implicit by sparse dim, or dynamic sizes, and not present as explicit dims.
+      Also see :func:`dim_tags_set`.
+    :rtype: set[_ImplicitDim]
+    """
+    self_dim_tags = set(self.dim_tags)
+    dims = set()
+    if self.sparse_dim and self.sparse_dim not in self_dim_tags:
+      dims.add(ImplicitSparseDim(self.sparse_dim))
+    for dim in self.dim_tags:
+      if dim.dyn_size_ext:
+        for dim_ in dim.dyn_size_ext.dim_tags:
+          if dim_ not in self_dim_tags:
+            dims.add(ImplicitDynSizeDim(dim_))
+    return dims
+
+  @property
+  def dim_tags_set_implicit_only(self):
+    """
+    :return: Dim tags implicit by sparse dim, or dynamic sizes, and not present as explicit dims.
+      Also see :func:`dim_tags_set`.
+    :rtype: set[DimensionTag]
+    """
+    return set(dim.tag for dim in self.dim_tags_set_implicit_only_wrapped)
+
+  @property
+  def dim_tags_set_implicit(self):
+    """
+    This is mostly intended to be used for verification, such as ``out_shape`` in a layer.
+      https://github.com/rwth-i6/returnn/issues/706
+
+    We return a set because when dim tags (dimensions, and the shape) are checked,
+    we never want that the order plays any role.
+      https://github.com/rwth-i6/returnn/wiki/RETURNN-principles
+    Further, dimension tags should ideally be unique.
+      https://github.com/rwth-i6/returnn/issues/632
+    (This is not enforced currently, but we should not treat this specially now.)
+
+    :return: set of dim tags
+    :rtype: set[DimensionTag]
+    """
+    dims = set(self.dim_tags)
+    dims.update(self.dim_tags_set_implicit_only)
+    return dims
+
   @property
   def ndim(self):
     """