diff --git a/requests/_internal_utils.py b/requests/_internal_utils.py
index 87e2ca00..759d9a56 100644
--- a/requests/_internal_utils.py
+++ b/requests/_internal_utils.py
@@ -8,7 +8,7 @@ Provides utility functions that are consumed internally by Requests
 which depend on extremely few external helpers (such as compat)
 """
 
-from .compat import is_py2, builtin_str
+from .compat import is_py2, builtin_str, str
 
 
 def to_native_string(string, encoding='ascii'):
@@ -25,3 +25,18 @@ def to_native_string(string, encoding='ascii'):
             out = string.decode(encoding)
 
     return out
+
+
+def unicode_is_ascii(u_string):
+    """Determine if unicode string only contains ASCII characters.
+
+    :param str u_string: unicode string to check. Must be unicode
+        and not Python 2 `str`.
+    :rtype: bool
+    """
+    assert isinstance(u_string, str)
+    try:
+        u_string.encode('ascii')
+        return True
+    except UnicodeEncodeError:
+        return False