diff --git a/tests.py b/tests.py
index 09a60ca..f5e8bcb 100644
--- a/tests.py
+++ b/tests.py
@@ -149,13 +149,6 @@ def setUp(self):
         snapshot system package list
         """
         self.dir = tempfile.mkdtemp()
-        self.no_global = (
-            "lib/python{}.{}"
-            "/no-global-site-packages.txt"
-        ).format(
-            sys.version_info.major,
-            sys.version_info.minor
-        )
 
     def tearDown(self):
         if os.path.exists(self.dir):
@@ -164,30 +157,50 @@ def tearDown(self):
     def test_system_site_packages(self):
         """
         test that creating a venv with system_site_packages=True
-        results in a venv that does not contain the no-global-site-packages file
-
+        results in a venv that does not have system packages installed
         """
+
         venv = VirtualEnvironment(self.dir, system_site_packages=True)
-        venv._create()
-        expected = os.path.join(venv.path, self.no_global)
-        self.assertTrue(
-            not os.path.exists(expected)
-        )
+        venv.open_or_create()
+
+        # always installed and accessible
+        self.assertTrue(venv.is_installed("pip"))
+        self.assertTrue(venv.is_installed("wheel"))
+        self.assertTrue(venv.is_installed("setuptools"))
+        self.assertTrue(venv.is_accessible("pip"))
+        self.assertTrue(venv.is_accessible("wheel"))
+        self.assertTrue(venv.is_accessible("setuptools"))
+
+        # system packages are not listed with -l option
+        self.assertFalse(venv.is_installed("six"))
+        self.assertFalse(venv.is_installed("virtualenv"))
+
+        # but they are accessible
+        self.assertTrue(venv.is_accessible("six"))
+        self.assertTrue(venv.is_accessible("virtualenv"))
 
     def test_no_system_site_packages(self):
         """
         test that creating a venv with system_site_packages=False
-        results in a venv that contains the no-global-site-packages
-        file
-
+        results in a venv that does not have system packages installed
         """
-        venv = VirtualEnvironment(self.dir)
-        venv._create()
-        expected = os.path.join(venv.path, self.no_global)
-        self.assertTrue(
-            os.path.exists(expected)
-        )
 
+        venv = VirtualEnvironment(self.dir)
+        venv.open_or_create()
+
+        # always installed and accessible
+        self.assertTrue(venv.is_installed("pip"))
+        self.assertTrue(venv.is_installed("wheel"))
+        self.assertTrue(venv.is_installed("setuptools"))
+        self.assertTrue(venv.is_accessible("pip"))
+        self.assertTrue(venv.is_accessible("wheel"))
+        self.assertTrue(venv.is_accessible("setuptools"))
+
+        # system packages are not accessible
+        self.assertFalse(venv.is_installed("six"))
+        self.assertFalse(venv.is_installed("virtualenv"))
+        self.assertFalse(venv.is_accessible("six"))
+        self.assertFalse(venv.is_accessible("virtualenv"))
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/virtualenvapi/manage.py b/virtualenvapi/manage.py
index 59e04e4..8281198 100644
--- a/virtualenvapi/manage.py
+++ b/virtualenvapi/manage.py
@@ -283,6 +283,21 @@ def is_installed(self, package):
         else:
             return pkg_tuple[0].lower() in self.installed_package_names
 
+    def is_accessible(self, package):
+        """Returns True if the given package (given in pip's package syntax or a
+        tuple of ('name', 'ver')) is accessible in the virtual environment."""
+        if isinstance(package, tuple):
+            package = '=='.join(package)
+        if package.endswith('.git'):
+            pkg_name = os.path.split(package)[1][:-4]
+            return pkg_name in self.all_package_names or \
+                    pkg_name.replace('_', '-') in self.all_package_names
+        pkg_tuple = split_package_name(package)
+        if pkg_tuple[1] is not None:
+            return pkg_tuple in self.all_packages
+        else:
+            return pkg_tuple[0].lower() in self.all_package_names
+
     def upgrade(self, package, force=False):
         """Shortcut method to upgrade a package. If `force` is set to True,
         the package and all of its dependencies will be reinstalled, otherwise
@@ -333,6 +348,22 @@ def installed_packages(self):
                 ['freeze'] + freeze_options).split(linesep))))
 
     @property
+    def all_packages(self):
+        """
+        List of all packages accessible in this environment in
+        the format [(name, ver), ..].
+        """
+
+        freeze_options = ['--all'] if self.pip_version >= (8, 1, 0) else []
+        return list(map(split_package_name, filter(None, self._execute_pip(
+                ['freeze'] + freeze_options).split(linesep))))
+    @property
     def installed_package_names(self):
         """List of all package names that are installed in this environment."""
         return [name.lower() for name, _ in self.installed_packages]
+
+    @property
+    def all_package_names(self):
+        """List of all package names that are accessible in this environment."""
+
+        return [name.lower() for name, _ in self.all_packages]