added a method to test credential.

src/main/java/org/kohsuke/github/GitHub.java

@@ -188,6 +188,18 @@ public class GitHub {
                 .with("public", isPublic ? 1 : 0).to(getApiURL("/repos/create"), JsonRepository.class).wrap(this);
     }
 
+    /**
+     * Ensures that the credential is valid.
+     */
+    public boolean isCredentialValid() throws IOException {
+        try {
+            retrieveWithAuth(getApiURL("/user/show"),JsonUser.class);
+            return true;
+        } catch (IOException e) {
+            return false;
+        }
+    }
+
     WebClient createWebClient() throws IOException {
         WebClient wc = new WebClient();
         wc.setJavaScriptEnabled(false);

src/test/java/org/kohsuke/AppTest.java

@@ -1,8 +1,6 @@
 package org.kohsuke;
 
-import junit.framework.Test;
 import junit.framework.TestCase;
-import junit.framework.TestSuite;
 import org.kohsuke.github.GHOrganization;
 import org.kohsuke.github.GHRepository;
 import org.kohsuke.github.GHTeam;
@@ -16,6 +14,11 @@ import java.util.Set;
  * Unit test for simple App.
  */
 public class AppTest extends TestCase {
+    public void testCredentialValid() throws IOException {
+        assertTrue(GitHub.connect().isCredentialValid());
+        assertFalse(GitHub.connect("totally","bogus").isCredentialValid());
+    }
+
     public void testApp() throws IOException {
         GitHub gitHub = GitHub.connect();
 //        testOrganization(gitHub);