diff --git a/benchmarks/fetch/webidl-is.mjs b/benchmarks/fetch/webidl-is.mjs
new file mode 100644
index 00000000000..937c1658e47
--- /dev/null
+++ b/benchmarks/fetch/webidl-is.mjs
@@ -0,0 +1,32 @@
+import { group, bench, run } from 'mitata'
+import { webidl } from '../../lib/web/fetch/webidl.js'
+import { FormData, Headers } from '../../index.js'
+
+function assert (value) {
+  if (!value) {
+    throw new TypeError('value is not truthy')
+  }
+}
+
+const isPrototypeOf = webidl.util.MakeTypeAssertion(FormData.prototype)
+const privatePropertyIn = webidl.is.FormData
+
+const fd = new FormData()
+const notFd = new Headers()
+
+group(() => {
+  bench('common (good) case - isPrototypeOf', () => {
+    assert(isPrototypeOf(fd))
+  })
+  bench('uncommon (bad) case - isPrototypeOf', () => {
+    assert(!isPrototypeOf(notFd))
+  })
+  bench('common (good) case - #symbol in fd', () => {
+    assert(privatePropertyIn(fd))
+  })
+  bench('uncommon (bad) case - #symbol in fd', () => {
+    assert(!privatePropertyIn(notFd))
+  })
+})
+
+await run()
diff --git a/lib/web/fetch/formdata.js b/lib/web/fetch/formdata.js
index ef45cefb369..563a50111d5 100644
--- a/lib/web/fetch/formdata.js
+++ b/lib/web/fetch/formdata.js
@@ -194,11 +194,16 @@ class FormData {
   static setFormDataState (formData, newState) {
     formData.#state = newState
   }
+
+  static isFormData (value) {
+    return value !== null && typeof value === 'object' && #state in value
+  }
 }
 
-const { getFormDataState, setFormDataState } = FormData
+const { getFormDataState, setFormDataState, isFormData } = FormData
 Reflect.deleteProperty(FormData, 'getFormDataState')
 Reflect.deleteProperty(FormData, 'setFormDataState')
+Reflect.deleteProperty(FormData, 'isFormData')
 
 iteratorMixin('FormData', FormData, getFormDataState, 'name', 'value')
 
@@ -256,6 +261,6 @@ function makeEntry (name, value, filename) {
   return { name, value }
 }
 
-webidl.is.FormData = webidl.util.MakeTypeAssertion(FormData.prototype)
+webidl.is.FormData = isFormData
 
 module.exports = { FormData, makeEntry, setFormDataState }
diff --git a/lib/web/fetch/request.js b/lib/web/fetch/request.js
index fec12b58308..00e49b4f475 100644
--- a/lib/web/fetch/request.js
+++ b/lib/web/fetch/request.js
@@ -864,15 +864,20 @@ class Request {
   static setRequestState (request, newState) {
     request.#state = newState
   }
+
+  static isRequest (value) {
+    return value !== null && typeof value === 'object' && #state in value
+  }
 }
 
-const { setRequestSignal, getRequestDispatcher, setRequestDispatcher, setRequestHeaders, getRequestState, setRequestState } = Request
+const { setRequestSignal, getRequestDispatcher, setRequestDispatcher, setRequestHeaders, getRequestState, setRequestState, isRequest } = Request
 Reflect.deleteProperty(Request, 'setRequestSignal')
 Reflect.deleteProperty(Request, 'getRequestDispatcher')
 Reflect.deleteProperty(Request, 'setRequestDispatcher')
 Reflect.deleteProperty(Request, 'setRequestHeaders')
 Reflect.deleteProperty(Request, 'getRequestState')
 Reflect.deleteProperty(Request, 'setRequestState')
+Reflect.deleteProperty(Request, 'isRequest')
 
 mixinBody(Request, getRequestState)
 
@@ -986,7 +991,7 @@ Object.defineProperties(Request.prototype, {
   }
 })
 
-webidl.is.Request = webidl.util.MakeTypeAssertion(Request.prototype)
+webidl.is.Request = isRequest
 
 // https://fetch.spec.whatwg.org/#requestinfo
 webidl.converters.RequestInfo = function (V, prefix, argument) {
diff --git a/lib/web/fetch/response.js b/lib/web/fetch/response.js
index 60cee8bb26e..3d7ead654b3 100644
--- a/lib/web/fetch/response.js
+++ b/lib/web/fetch/response.js
@@ -297,13 +297,18 @@ class Response {
   static setResponseState (response, newState) {
     response.#state = newState
   }
+
+  static isResponse (value) {
+    return value !== null && typeof value === 'object' && #state in value
+  }
 }
 
-const { getResponseHeaders, setResponseHeaders, getResponseState, setResponseState } = Response
+const { getResponseHeaders, setResponseHeaders, getResponseState, setResponseState, isResponse } = Response
 Reflect.deleteProperty(Response, 'getResponseHeaders')
 Reflect.deleteProperty(Response, 'setResponseHeaders')
 Reflect.deleteProperty(Response, 'getResponseState')
 Reflect.deleteProperty(Response, 'setResponseState')
+Reflect.deleteProperty(Response, 'isResponse')
 
 mixinBody(Response, getResponseState)
 
@@ -619,7 +624,7 @@ webidl.converters.ResponseInit = webidl.dictionaryConverter([
   }
 ])
 
-webidl.is.Response = webidl.util.MakeTypeAssertion(Response.prototype)
+webidl.is.Response = isResponse
 
 module.exports = {
   isNetworkError,
diff --git a/lib/web/websocket/stream/websocketerror.js b/lib/web/websocket/stream/websocketerror.js
index e47551a96b9..f3d81dc36e9 100644
--- a/lib/web/websocket/stream/websocketerror.js
+++ b/lib/web/websocket/stream/websocketerror.js
@@ -62,10 +62,15 @@ class WebSocketError extends DOMException {
     error.#reason = reason
     return error
   }
+
+  static isWebSocketError (value) {
+    return value !== null && typeof value === 'object' && #closeCode in value
+  }
 }
 
-const { createUnvalidatedWebSocketError } = WebSocketError
-delete WebSocketError.createUnvalidatedWebSocketError
+const { createUnvalidatedWebSocketError, isWebSocketError } = WebSocketError
+Reflect.deleteProperty(WebSocketError, 'createUnvalidatedWebSocketError')
+Reflect.deleteProperty(WebSocketError, 'isWebSocketError')
 
 Object.defineProperties(WebSocketError.prototype, {
   closeCode: kEnumerableProperty,
@@ -78,6 +83,6 @@ Object.defineProperties(WebSocketError.prototype, {
   }
 })
 
-webidl.is.WebSocketError = webidl.util.MakeTypeAssertion(WebSocketError.prototype)
+webidl.is.WebSocketError = isWebSocketError
 
 module.exports = { WebSocketError, createUnvalidatedWebSocketError }