/etc/redhat-release will usually give this information. If you don't see an update version, you might still be the stock version you installed with.
And yes, Red Hat and Novell both use service packs, other wise just known as updates.
Like CentOS versions, say you installed CentOS 4.0. 4.1 would be CentOS 4.0 with Update 1. 4.5 would be CentOS 4.0 with Update 5. They name the Version after the OS and Update released by Red Hat.
It's a clone of Red Hat but I do believe they put whatever update you are currently on in the /etc/redhat-release. In some cases there might also be a /etc/redhat-version, but that is only Red Hat related and I believe they did that on older releases, before 4.
But when you hear a vendor ask "Service Pack", in Red Hat world, that means Update and in Novell world, it actually means Service Pack.
It's not only a Microsoft term, but I'm sure they've patented it and will be coming after all us Linux users sooner or later, cause their schmucks.