We read the documentation that is available on a service pack, paying special attention to the release notes which are a list of all known bugs and issues identified with the service pack when it was released.  In addition, we check the Knowledge Base articles at http://support.microsoft.com in order to identify issues that have surfaced after it was released. Specifically, attention is paid to the following issues:

• Hardware incompatibilities - These include RAID controllers, NIC cards, peripherals, specific BIOS or motherboard issues, display drivers, printers,  Hardware that is on Microsoft's hardware compatibility list (HCL) for each operating system should have been part of the service pack testing phase and should be less likely (theoretically) to have post release issues than other products.
• Software issues - Because service packs update so many DLL's, registry entries, and other core system components, they also have the capability of causing issues with many applications including Novell Clients, graphics programs, antivirus software and more.
• Issues with other application service packs - On occasion, Microsoft's operating system service packs cause incompatibilities with application service packs for SQL, Exchange, SMS, IIS, or other BackOffice products. In some cases, application service packs may need to be reinstalled after an OS service pack so be sure to check for these issues.
• Issues with Domain Controllers - Changes and updates in group policy, replication, network components and other updates can cause a variety of issues with domain controllers, whether or not they are running Active Directory.