diff --git a/src/monty/io.py b/src/monty/io.py
index 52bf2280..7e698d11 100644
--- a/src/monty/io.py
+++ b/src/monty/io.py
@@ -54,6 +54,40 @@ def zopen(filename: Union[str, Path], *args, **kwargs) -> IO:
     return open(filename, *args, **kwargs)
 
 
+def _get_line_ending(
+    file: str | Path | io.TextIOWrapper,
+) -> Literal["\r\n", "\n", "\r"]:
+    """Helper function to get line ending of a file.
+
+    This function assumes the file has a single consistent line ending.
+
+    Returns:
+        "\n": Unix line ending.
+        "\r\n": Windows line ending.
+        "\r": Classic MacOS line ending.
+
+    Raises:
+        ValueError: If file is empty or line ending is unknown.
+    """
+    if isinstance(file, (str, Path)):
+        with open(file, "rb") as f:
+            first_line = f.readline()
+    elif isinstance(file, io.TextIOWrapper):
+        first_line = file.buffer.readline()
+
+    if not first_line:
+        raise ValueError("empty file.")
+
+    if first_line.endswith(b"\r\n"):
+        return "\r\n"
+    elif first_line.endswith(b"\n"):
+        return "\n"
+    elif first_line.endswith(b"\r"):
+        return "\r"
+    else:
+        raise ValueError(f"Unknown line ending in file {repr(first_line)}.")
+
+
 def reverse_readfile(
     filename: Union[str, Path],
     l_end: Literal["AUTO"] | str = "AUTO",
diff --git a/tests/test_io.py b/tests/test_io.py
index 9daa17be..d55b9ad9 100644
--- a/tests/test_io.py
+++ b/tests/test_io.py
@@ -8,6 +8,7 @@
 from monty.io import (
     FileLock,
     FileLockException,
+    _get_line_ending,
     reverse_readfile,
     reverse_readline,
     zopen,
@@ -17,6 +18,43 @@
 TEST_DIR = os.path.join(os.path.dirname(__file__), "test_files")
 
 
+class TestGetLineEnding:
+    @pytest.mark.parametrize("l_end", ["\n", "\r\n", "\r"])
+    def test_get_line_ending(self, l_end):
+        """Test file with:
+        Unix line ending (\n)
+        Windows line ending (\r\n)
+        Classic MacOS line ending (\r)
+        """
+        with ScratchDir("."):
+            test_file = "test_file.txt"
+            with open(test_file, "wb") as f:
+                f.write(f"This is a test{l_end}Second line{l_end}".encode())
+
+            assert _get_line_ending(test_file) == l_end
+            assert _get_line_ending(Path(test_file)) == l_end
+
+            with open(test_file, "r") as f:
+                assert _get_line_ending(f) == l_end
+
+    def test_empty_file(self):
+        with ScratchDir("."):
+            test_file = "empty_file.txt"
+            open(test_file, "w").close()
+
+            with pytest.raises(ValueError, match="empty file"):
+                _get_line_ending(test_file)
+
+    def test_unknown_line_ending(self):
+        with ScratchDir("."):
+            test_file = "test_unknown.txt"
+            with open(test_file, "wb") as f:
+                f.write(b"This is a test\036")
+
+            with pytest.raises(ValueError, match="Unknown line ending"):
+                _get_line_ending(test_file)
+
+
 class TestReverseReadline:
     NUMLINES = 3000