If you registered in the past and are still using the same machine:
In ISE Project Navigator or EDK XPS, select Help -> About.
A pop-up box is displayed with the Registration ID.
In addition, you can try the following:
For version 10.1 and earlier, entitlements can be found in the Xilinx Licensing Center here:
If you have already licensed an earlier version, your Registration ID can be found under the Legacy Licensing tab in the Entitlement Center.
For additional license inquiries please see the Xilinx Licensing FAQ: http://www.xilinx.com/tools/faq.htm