MatrixQ

MatrixQ[expr] gives True if expr is a rectangular list of lists representing a matrix.

MatrixQ[expr, test] gives True only if test yields True for each matrix element.

Examples

Test for matrices:

MatrixQ[{{1, 2}, {3, 4}}]
(* True *)

Non-rectangular arrays are not matrices:

MatrixQ[{{1, 2}, {3}}]
(* False *)

Test for numeric matrices:

MatrixQ[{{1, 2}, {3, 4}}, IntegerQ]
(* True *)

Please visit the official Wolfram Language Reference for more details.