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
6. Troubleshooting Active Directory Issues
http://blogs.dirteam.com/blogs/paulbergson/archive/2009/01/26/troubleshooting-active-directory-issues.aspx
Leonardo Côco edited Original. Comment: Added tag
i.biswajith, below link is best suitable in point 2.
social.technet.microsoft.com/.../584.active-directory-replication-over-firewalls.aspx