diff --git a/_e2e/fn_test.ts b/_e2e/fn_test.ts
index acfd827..8a7fc66 100644
--- a/_e2e/fn_test.ts
+++ b/_e2e/fn_test.ts
@@ -1,5 +1,5 @@
 // Copyright 2021-Present the Unitest authors. All rights reserved. MIT license.
-import { expect, fn, test } from "../mod.ts";
+import { expect, fn, isMockObject, test } from "../mod.ts";
 
 test("should define implementation as default", () => {
   const mockObject = fn().defaultImplementation(() => true);
@@ -66,3 +66,9 @@ test("should clear mock and all registered once implementations and default", ()
   mockObject();
   expect(mockObject).toHaveReturnedWith(undefined);
 });
+
+test("should be mock object", () => {
+  const mockObject = fn();
+  expect(isMockObject(mockObject)).toBeTruthy();
+  expect(isMockObject({})).toBeFalsy();
+});
diff --git a/mock/README.md b/mock/README.md
index b59b1cb..8e6520e 100644
--- a/mock/README.md
+++ b/mock/README.md
@@ -42,6 +42,25 @@ test("plays video", () => {
 });
 ```
 
+## isMockObject
+
+Whatever argument is `MockObject` or not.
+
+```ts
+import {
+  expect,
+  fn,
+  isMockObject,
+  test,
+} from "https://deno.land/x/unitest@$VERSION/mod.ts";
+
+test("should be mock object", () => {
+  const mockObject = fn();
+  expect(isMockObject(mockObject)).toBeTruthy();
+  expect(isMockObject({})).toBeFalsy();
+});
+```
+
 ## mockObject.mock.calls
 
 An array containing the call arguments of all calls that have been made to this
diff --git a/mock/fn.ts b/mock/fn.ts
index c3d57be..3b37a5a 100644
--- a/mock/fn.ts
+++ b/mock/fn.ts
@@ -1,6 +1,8 @@
 // Copyright 2021-Present the Unitest authors. All rights reserved. MIT license.
 // This module is browser compatible.
 
+import { isObject } from "../deps.ts";
+import { prop } from "../matcher/utils.ts";
 import { Mock } from "./mock.ts";
 import type { MockSpec } from "./mock.ts";
 import { incrementalNumber } from "./utils.ts";
@@ -328,5 +330,32 @@ function fn(
   return call as MockObject;
 }
 
-export { fn, MockFnStore };
+/** Whatever argument is `MockObject` or not.
+ * ```ts
+ * import {
+ *   expect,
+ *   fn,
+ *   isMockObject,
+ *   test,
+ * } from "https://deno.land/x/unitest@$VERSION/mod.ts";
+ *
+ * test("should be mock object", () => {
+ *   const mockObject = fn();
+ *   expect(isMockObject(mockObject)).toBeTruthy();
+ *   expect(isMockObject({})).toBeFalsy();
+ * });
+ * ```
+ */
+function isMockObject<A extends readonly unknown[] = any[], R = unknown>(
+  value: object,
+): value is MockObject<A, R> {
+  const mock = prop("mock", value);
+  if (!isObject(mock)) return false;
+
+  return ["results", "calls", "callOrderNumbers"].every((key) =>
+    Array.isArray(prop(key, mock))
+  );
+}
+
+export { fn, isMockObject, MockFnStore };
 export type { MockObject };
diff --git a/mock/fn_test.ts b/mock/fn_test.ts
index 7378b09..98e66f6 100644
--- a/mock/fn_test.ts
+++ b/mock/fn_test.ts
@@ -1,6 +1,6 @@
 // Copyright 2021-Present the Unitest authors. All rights reserved. MIT license.
 
-import { fn, MockFnStore } from "./fn.ts";
+import { fn, isMockObject, MockFnStore } from "./fn.ts";
 import { isFunction } from "../deps.ts";
 import { expect } from "../expect/mod.ts";
 import { assert, assertEquals, assertExists } from "../dev_deps.ts";
@@ -314,3 +314,42 @@ Deno.test("reset should clear once implementation and default", () => {
   assertEquals(mockObject(), undefined);
   assertEquals(mockObject(), undefined);
 });
+
+Deno.test("isMockObject", () => {
+  const table: [
+    ...Parameters<typeof isMockObject>,
+    ReturnType<typeof isMockObject>,
+  ][] = [
+    [{}, false],
+    [[], false],
+    [{ mock: {} }, false],
+    [{
+      mock: {
+        results: [],
+        callOrderNumbers: [],
+      },
+    }, false],
+    [{
+      mock: {
+        callOrderNumbers: [],
+      },
+    }, false],
+    [{
+      mock: {
+        calls: [],
+        results: [],
+        callOrderNumbers: [],
+      },
+    }, true],
+    [{
+      mock: {
+        calls: [[1]],
+        results: [{ type: "return", value: 1 }],
+        callOrderNumbers: [1],
+      },
+    }, true],
+    [fn(), true],
+  ];
+
+  table.forEach(([value, result]) => assertEquals(isMockObject(value), result));
+});