how do i evaluate a patch ?

microsoft releases many patches periodically . how do i evaluate these patches after applying them to servers. for : EX we have latest patches like MS03-026. MS03-027 & 028 . i have applied patch from MS03-026 to win2k server & now i want to test what this patch does & its upshot before i apply this patch to all the servers.

how do i test a patch ??