UPDATE: now deals properly with proxies.

The following extension methods to ISession allow finding if a property or entity has changed, and the original value of a property.

public static class SessionExtensions

{

    public static Boolean IsDirtyEntity(this ISession session, Object entity)

    {

        ISessionImplementor sessionImpl = session.GetSessionImplementation();

        IPersistenceContext persistenceContext = sessionImpl.PersistenceContext;

        EntityEntry oldEntry = persistenceContext.GetEntry(entity);

        String className = oldEntry.EntityName;

        IEntityPersister persister = sessionImpl.Factory.GetEntityPersister(className);

        if ((oldEntry == null) && (entity is INHibernateProxy))

        {

            INHibernateProxy proxy = entity as INHibernateProxy;

            Object obj = sessionImpl.PersistenceContext.Unproxy(proxy);

            oldEntry = sessionImpl.PersistenceContext.GetEntry(obj);

        }

        Object [] oldState = oldEntry.LoadedState;

        Object [] currentState = persister.GetPropertyValues(entity, sessionImpl.EntityMode);

     Int32 [] dirtyProps = oldState.Select((o, i) => (oldState [ i ] == currentState [ i ]) ? -1 : i).Where(x => x >= 0).ToArray();

        return (dirtyProps != null);

    }

    public static Boolean IsDirtyProperty(this ISession session, Object entity, String propertyName)

    {

        ISessionImplementor sessionImpl = session.GetSessionImplementation();

        IPersistenceContext persistenceContext = sessionImpl.PersistenceContext;

        EntityEntry oldEntry = persistenceContext.GetEntry(entity);

        String className = oldEntry.EntityName;

        IEntityPersister persister = sessionImpl.Factory.GetEntityPersister(className);

        if ((oldEntry == null) && (entity is INHibernateProxy))

        {

            INHibernateProxy proxy = entity as INHibernateProxy;

            Object obj = sessionImpl.PersistenceContext.Unproxy(proxy);

            oldEntry = sessionImpl.PersistenceContext.GetEntry(obj);

        }

        Object [] oldState = oldEntry.LoadedState;

        Object [] currentState = persister.GetPropertyValues(entity, sessionImpl.EntityMode);

        Int32 [] dirtyProps = persister.FindDirty(currentState, oldState, entity, sessionImpl);

        Int32 index = Array.IndexOf(persister.PropertyNames, propertyName);

        Boolean isDirty = (dirtyProps != null) ? (Array.IndexOf(dirtyProps, index) != -1) : false;

        return (isDirty);

    }

    public static Object GetOriginalEntityProperty(this ISession session, Object entity, String propertyName)

    {

        ISessionImplementor sessionImpl = session.GetSessionImplementation();

        IPersistenceContext persistenceContext = sessionImpl.PersistenceContext;

        EntityEntry oldEntry = persistenceContext.GetEntry(entity);

        String className = oldEntry.EntityName;

        IEntityPersister persister = sessionImpl.Factory.GetEntityPersister(className);

        if ((oldEntry == null) && (entity is INHibernateProxy))

        {

            INHibernateProxy proxy = entity as INHibernateProxy;

            Object obj = sessionImpl.PersistenceContext.Unproxy(proxy);

            oldEntry = sessionImpl.PersistenceContext.GetEntry(obj);

        }

        Object [] oldState = oldEntry.LoadedState;

        Object [] currentState = persister.GetPropertyValues(entity, sessionImpl.EntityMode);

        Int32 [] dirtyProps = persister.FindDirty(currentState, oldState, entity, sessionImpl);

        Int32 index = Array.IndexOf(persister.PropertyNames, propertyName);

        Boolean isDirty = (dirtyProps != null) ? (Array.IndexOf(dirtyProps, index) != -1) : false;

        return ((isDirty == true) ? oldState [ index ] : currentState [ index ]);

    }

}