diff --git a/docs/topics/contracts.rst b/docs/topics/contracts.rst
index 70f20d4ed..9337375bb 100644
--- a/docs/topics/contracts.rst
+++ b/docs/topics/contracts.rst
@@ -120,3 +120,23 @@ get the failures pretty printed::
             for header in self.args:
                 if header not in response.headers:
                     raise ContractFail('X-CustomHeader not present')
+
+
+Detecting check runs
+====================
+
+When ``scrapy check`` is running, the ``SCRAPY_CHECK`` environment variable is
+set to the ``true`` string. You can use `os.environ`_ to perform any change to
+your spiders or your settings when ``scrapy check`` is used::
+
+    import os
+    import scrapy
+
+    class ExampleSpider(scrapy.Spider):
+        name = 'example'
+
+        def __init__(self):
+            if os.environ.get('SCRAPY_CHECK'):
+                pass  # Do some scraper adjustments when a check is running
+
+.. _os.environ: https://docs.python.org/3/library/os.html#os.environ
diff --git a/scrapy/commands/check.py b/scrapy/commands/check.py
index b8a9ef989..ab73e85e7 100644
--- a/scrapy/commands/check.py
+++ b/scrapy/commands/check.py
@@ -6,7 +6,7 @@ from unittest import TextTestRunner, TextTestResult as _TextTestResult
 
 from scrapy.commands import ScrapyCommand
 from scrapy.contracts import ContractsManager
-from scrapy.utils.misc import load_object
+from scrapy.utils.misc import load_object, set_environ
 from scrapy.utils.conf import build_component_list
 
 
@@ -68,16 +68,17 @@ class Command(ScrapyCommand):
 
         spider_loader = self.crawler_process.spider_loader
 
-        for spidername in args or spider_loader.list():
-            spidercls = spider_loader.load(spidername)
-            spidercls.start_requests = lambda s: conman.from_spider(s, result)
+        with set_environ(SCRAPY_CHECK='true'):
+            for spidername in args or spider_loader.list():
+                spidercls = spider_loader.load(spidername)
+                spidercls.start_requests = lambda s: conman.from_spider(s, result)
 
-            tested_methods = conman.tested_methods_from_spidercls(spidercls)
-            if opts.list:
-                for method in tested_methods:
-                    contract_reqs[spidercls.name].append(method)
-            elif tested_methods:
-                self.crawler_process.crawl(spidercls)
+                tested_methods = conman.tested_methods_from_spidercls(spidercls)
+                if opts.list:
+                    for method in tested_methods:
+                        contract_reqs[spidercls.name].append(method)
+                elif tested_methods:
+                    self.crawler_process.crawl(spidercls)
 
         # start checks
         if opts.list:
diff --git a/scrapy/utils/misc.py b/scrapy/utils/misc.py
index ddaa7f7bf..f638adb25 100644
--- a/scrapy/utils/misc.py
+++ b/scrapy/utils/misc.py
@@ -1,6 +1,8 @@
 """Helper functions which don't fit anywhere else"""
+import os
 import re
 import hashlib
+from contextlib import contextmanager
 from importlib import import_module
 from pkgutil import iter_modules
 
@@ -142,3 +144,21 @@ def create_instance(objcls, settings, crawler, *args, **kwargs):
         return objcls.from_settings(settings, *args, **kwargs)
     else:
         return objcls(*args, **kwargs)
+
+
+@contextmanager
+def set_environ(**kwargs):
+    """Temporarily set environment variables inside the context manager and
+    fully restore previous environment afterwards
+    """
+
+    original_env = {k: os.environ.get(k) for k in kwargs}
+    os.environ.update(kwargs)
+    try:
+        yield
+    finally:
+        for k, v in original_env.items():
+            if v is None:
+                del os.environ[k]
+            else:
+                os.environ[k] = v
diff --git a/tests/test_utils_misc/__init__.py b/tests/test_utils_misc/__init__.py
index fcb7772ab..e109d5343 100644
--- a/tests/test_utils_misc/__init__.py
+++ b/tests/test_utils_misc/__init__.py
@@ -3,12 +3,13 @@ import os
 import unittest
 
 from scrapy.item import Item, Field
-from scrapy.utils.misc import arg_to_iter, create_instance, load_object, walk_modules
+from scrapy.utils.misc import arg_to_iter, create_instance, load_object, set_environ, walk_modules
 
 from tests import mock
 
 __doctests__ = ['scrapy.utils.misc']
 
+
 class UtilsMiscTestCase(unittest.TestCase):
 
     def test_load_object(self):
@@ -130,5 +131,18 @@ class UtilsMiscTestCase(unittest.TestCase):
         with self.assertRaises(ValueError):
             create_instance(m, None, None)
 
+    def test_set_environ(self):
+        assert os.environ.get('some_test_environ') is None
+        with set_environ(some_test_environ='test_value'):
+            assert os.environ.get('some_test_environ') == 'test_value'
+        assert os.environ.get('some_test_environ') is None
+
+        os.environ['some_test_environ'] = 'test'
+        assert os.environ.get('some_test_environ') == 'test'
+        with set_environ(some_test_environ='test_value'):
+            assert os.environ.get('some_test_environ') == 'test_value'
+        assert os.environ.get('some_test_environ') == 'test'
+
+
 if __name__ == "__main__":
     unittest.main()