There are two types of arguments for services:

• Arguments that were passed on the process start command line. You can get to those easily using Process Explorer, etc.
• Arguments that were passed to the ServiceMain function. This is the WIndows API that a service is supposed to implement. The .NET equivalent is ServiceBase.OnStart. This is what is used when you do an SC START \[arguments\]. This has nothing to do with "command line process arguments".

The second type of parameters is probably only known by the service itself, if the implementation makes any use of it which is not the case for many services. I don't think Windows keep track of this when we look at low level Windows structures like the PEB: Process and Thread Structures (MSDN), even the undocumented parts of it, Undocumented functions of NTDLL.