Is there a way to test RAM on a Windows 2008 R2 x64 machine?
Unless you have a manufacturer-provided diagnostic tool (that you trust) you will probably need to take an outage window and run something like Memtest86 (http://www.memtest86.com/).
If you suspect bad RAM I would advise runing Memtest86 anyway regardless of what any manufacturer diagnostics say.
Windows Server 2008 (and Vista/7) have a built in memory testing tool. It requires a reboot. Type "memory" into the start menu and it will be one of the first returns.