diff --git a/src/debug.rs b/src/debug.rs
new file mode 100644
index 0000000..491ffdf
--- /dev/null
+++ b/src/debug.rs
@@ -0,0 +1,28 @@
+//! Debugging APIs: these are meant for use when unit-testing or
+//! debugging your application but aren't ordinarily needed.
+
+use crate::Database;
+use crate::Query;
+use crate::QueryStorageOps;
+use crate::QueryTable;
+
+pub trait DebugQueryTable {
+    type Key;
+
+    /// True if salsa thinks that the value for `key` is a
+    /// **constant**, meaning that it can never change, no matter what
+    /// values the inputs take on from this point.
+    fn is_constant(&self, key: Self::Key) -> bool;
+}
+
+impl<DB, Q> DebugQueryTable for QueryTable<'_, DB, Q>
+where
+    DB: Database,
+    Q: Query<DB>,
+{
+    type Key = Q::Key;
+
+    fn is_constant(&self, key: Q::Key) -> bool {
+        self.storage.is_constant(self.db, &key)
+    }
+}
diff --git a/src/derived.rs b/src/derived.rs
index 5a37097..c1042f3 100644
--- a/src/derived.rs
+++ b/src/derived.rs
@@ -396,6 +396,15 @@ where
 
         true
     }
+
+    fn is_constant(&self, _db: &DB, key: &Q::Key) -> bool {
+        let map_read = self.map.read();
+        match map_read.get(key) {
+            None => false,
+            Some(QueryState::InProgress) => panic!("query in progress"),
+            Some(QueryState::Memoized(memo)) => memo.changed_at.is_constant(),
+        }
+    }
 }
 
 impl<DB, Q, MP> UncheckedMutQueryStorageOps<DB, Q> for DerivedStorage<DB, Q, MP>
diff --git a/src/input.rs b/src/input.rs
index c81523a..face524 100644
--- a/src/input.rs
+++ b/src/input.rs
@@ -177,6 +177,14 @@ where
 
         changed_at.changed_since(revision)
     }
+
+    fn is_constant(&self, _db: &DB, key: &Q::Key) -> bool {
+        let map_read = self.map.read();
+        map_read
+            .get(key)
+            .map(|v| v.changed_at.is_constant())
+            .unwrap_or(false)
+    }
 }
 
 impl<DB, Q> InputQueryStorageOps<DB, Q> for InputStorage<DB, Q>
diff --git a/src/lib.rs b/src/lib.rs
index cb98a17..af20b33 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -12,6 +12,7 @@ use std::fmt::Display;
 use std::fmt::Write;
 use std::hash::Hash;
 
+pub mod debug;
 pub mod derived;
 pub mod input;
 pub mod runtime;
@@ -119,6 +120,9 @@ where
         key: &Q::Key,
         descriptor: &DB::QueryDescriptor,
     ) -> bool;
+
+    /// Check if `key` is (currently) believed to be a constant.
+    fn is_constant(&self, db: &DB, key: &Q::Key) -> bool;
 }
 
 /// An optional trait that is implemented for "user mutable" storage: