Developer`PackedArrayQ

PackedArrayQ[expr] returns True if expr is a packed array in its internal representation, and returns False otherwise.

PackedArrayQ[expr, type] returns True if expr is a packed array of objects of the specified type.

PackedArrayQ[expr, type, rank] returns True if expr is a packed array of the specified rank.

Supported types are Integer, Real and Complex.

See also: ToPackedArray, ByteCount.