diff --git a/tools/build.rs b/tools/build.rs
index 608f687..b3b1113 100644
--- a/tools/build.rs
+++ b/tools/build.rs
@@ -13,10 +13,12 @@ fn main() {
     if flagfile.exists() {
         subplot_build::codegen("subplot/opgpcard.subplot")
             .expect("failed to generate code with Subplot");
+        println!("cargo:warning=generating subplot tests");
     } else {
         // If we're not generating code from the subplot, we should at
         // least create an empty file so that the tests/opgpcard.rs
         // file can include it. Otherwise the build will fail.
+        println!("cargo:warning=flagfile not found");
         let out_dir = std::env::var("OUT_DIR").unwrap();
         let include = Path::new(&out_dir).join("opgpcard.rs");
         eprintln!("build.rs: include={}", include.display());