diff --git a/sections/advanced.md b/sections/advanced.md
index 442dbff1..02c637ea 100644
--- a/sections/advanced.md
+++ b/sections/advanced.md
@@ -885,3 +885,8 @@ tag or SHA) and will cache based on that.  Using a branch name (or `HEAD`) for
 the value of [`rev`](#repos-rev) is not supported and will only represent the state of
 that mutable ref at the time of hook installation (and will *NOT* update
 automatically).
+
+## Detecting when `pre-commit` is running
+
+You can check for the existence of the `PRE_COMMIT` environment variable within
+a hook to determine if it's being run from `pre-commit`.