Options:

Use JCR Directly

Client code example:

Node node = (Node) session.getItem("/foo/bar");
Node content = node.getNode("jcr:content");
long lastModified = content.getProperty("jcr:lastModified").getLong()

Lenya Publication API

Client code example:

Document doc = session.getDocument("/foo/bar");
long lastModified = doc.getLastModified();

Via Sling