diff --git a/src/check.rs b/src/check.rs
index e72b5e2..3cc1269 100644
--- a/src/check.rs
+++ b/src/check.rs
@@ -7,6 +7,8 @@ use crate::integer_math::{mod_mul, mod_pow};
 /// Does trial division with a small wheel up to `log2(n)` and then uses a
 /// deterministic Miller-Rabin primality test.
 ///
+/// If the `fast_test` feature is enabled this function instead calls [`machine_prime::is_prime`] with the `small` feature.
+///
 /// # Example
 ///
 /// Basic usage: