diff --git a/sud.c b/sud.c
index 3d2bb12..364a9ba 100644
--- a/sud.c
+++ b/sud.c
@@ -241,3 +241,14 @@ enum check_res check(const struct sudoku *sud)
 	/* If we've got this far, all is well. */
 	return SOLVED;
 }
+
+bool filled(const struct sudoku *sud)
+{
+	for (unsigned r = 0; r < NDIGITS; ++r) {
+		for (unsigned c = 0; c < NDIGITS; ++c) {
+			if (!sud->cells[r][c].det)
+				return false;
+		}
+	}
+	return true;
+}
diff --git a/sud.h b/sud.h
index bdb8971..53c159f 100644
--- a/sud.h
+++ b/sud.h
@@ -71,6 +71,11 @@ void print(const struct sudoku *sud);
  * Determine whether the sudoku has been solved correctly, contains
  * invalid choices or is incomplete.
  */
-enum check_res check(const struct sudoku *sud);    
+enum check_res check(const struct sudoku *sud);
+
+/**
+ * Determine whether all the sudoku's cells have been determined.
+ */
+bool filled(const struct sudoku *sud);
 
 #endif