1. First try to ping the DC. Second are you able to ping the DSA GUID?If the DC is pingable but GUID is not pinging , see the below link for that.

http://www.windowsitpro.com/blog/active-directory-troubleshooting-tips-and-tricks-blog-34/active-directory/how-dcs-find-one-another-137278

2. Some ports should be open for AD replication, see the below link for that.

http://technet.microsoft.com/en-us/library/dd772723(WS.10).aspx 

3. Use the PortQry for port Troubleshooting. You can use a GUI tool(portqueryui.exe).

http://support.microsoft.com/kb/310099

4. Repadmin command for experts, you can use the repadmin for Replication Troubleshooting:

http://technet.microsoft.com/en-us/library/cc811549(WS.10).aspx

5. Lingering objects prevent Active Directory replication from occurring

6. Troubleshooting Active Directory Issues  

http://blogs.dirteam.com/blogs/paulbergson/archive/2009/01/26/troubleshooting-active-directory-issues.aspx


7. Use Repadmin for finding replication issue.