diff --git a/sud.c b/sud.c
index 5b6d331..aabe37e 100644
--- a/sud.c
+++ b/sud.c
@@ -171,3 +171,64 @@ void print(const struct sudoku *sud)
 		putchar('\n');
 	}
 }
+
+static void zerocounts(unsigned counts[NDIGITS])
+{
+	for (unsigned i = 0; i < NDIGITS; ++i)
+		counts[i] = 0;
+}
+
+static bool checkcounts(unsigned counts[NDIGITS])
+{
+	for (unsigned i = 0; i < NDIGITS; ++i) {
+		if (counts[i] != 1)
+			return false;
+	}
+	return true;
+}
+
+enum check_res check(const struct sudoku *sud)
+{
+	unsigned r, c, i, j, digitcounts[NDIGITS];
+
+	/* Check each row. */
+	for (r = 0; r < NDIGITS; ++r) {
+		zerocounts(digitcounts);
+		for (c = 0; c < NDIGITS; ++c) {
+			if (!sud->cells[r][c].det)
+				return INCOMPLETE;
+			++digitcounts[sud->cells[r][c].val];
+		}
+		if (!checkcounts(digitcounts))
+			return INCORRECT;
+	}
+
+	/* Check each column. */
+	for (c = 0; c < NDIGITS; ++c) {
+		zerocounts(digitcounts);
+		for (r = 0; r < NDIGITS; ++r) {
+			if (!sud->cells[r][c].det)
+				return INCOMPLETE;
+			++digitcounts[sud->cells[r][c].val];
+		}
+		if (!checkcounts(digitcounts))
+			return INCORRECT;
+	}
+
+	/* Check each segment. */
+	for (i = 0; i < NDIGITS; ++i) {
+		zerocounts(digitcounts);
+		for (j = 0; j < NDIGITS; ++j) {
+			r = 3 * (i / 3) + j / 3;
+			c = 3 * (i % 3) + j % 3;
+			if (!sud->cells[r][c].det)
+				return INCOMPLETE;
+			++digitcounts[sud->cells[r][c].val];
+		}
+		if (!checkcounts(digitcounts))
+			return INCORRECT;
+	}
+
+	/* If we've got this far, all is well. */
+	return SOLVED;
+}
diff --git a/sud.h b/sud.h
index 40178a7..df07d4b 100644
--- a/sud.h
+++ b/sud.h
@@ -22,7 +22,7 @@
 #include <stdbool.h>
 
 #define SEGLEN 3
-#define NDIGITS 9
+#define NDIGITS (SEGLEN * SEGLEN)
 
 struct cellstate {
 	bool det;
@@ -38,6 +38,8 @@ struct sudoku {
 
 enum update_res { NOT_ALLOWED, ALREADY_DET, OK };
 
+enum check_res { INCOMPLETE, INCORRECT, SOLVED };
+
 /**
  * Populate the sudoku with some random values.
  */
@@ -57,4 +59,10 @@ update(struct sudoku *sud, unsigned r, unsigned c, unsigned val);
  */
 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);    
+
 #endif