🚨 add warning if format cant be guessed

Author: MarcoGorelli

pandas/core/tools/datetimes.py

@@ -132,7 +132,16 @@ def _guess_datetime_format_for_array(arr, dayfirst: bool | None = False) -> str
     if (first_non_null := tslib.first_non_null(arr)) != -1:
         if type(first_non_nan_element := arr[first_non_null]) is str:
             # GH#32264 np.str_ object
-            return guess_datetime_format(first_non_nan_element, dayfirst=dayfirst)
+            guessed_format = guess_datetime_format(
+                first_non_nan_element, dayfirst=dayfirst
+            )
+            if guessed_format is not None:
+                return guessed_format
+            warnings.warn(
+                "Could not infer format - "
+                "to ensure consistent parsing, specify a format.",
+                stacklevel=find_stack_level(),
+            )
     return None