There are people that work in the field and journals that publish that kind of information. Part of the process would be a formal description of the plant and its location. If its a cross between and arach and a non arach, then your could call it an arach hybrid. Keep in mind that if it is a cross between an arach and another semp species, for example, S. montanum, then you could also call it a montanum hybrid.
At one point in the past, nomenclature rules permitted that assignment of new names to species crosses. I can't think of an example at the moment, but some of the names that were assigned to semps that look like species names are actually hybrids. Since semps have been cultivated by people for many years, it can be very difficult to know whether something is a hybrid or not, new or not, etc. Fortunately, records of a lot of this have been kept over the years, so there is some history for a lot of varieties.
I tried several times over the past few years to use the Internet to build just a list of species names. However, I have never been able to complete that project and eventually gave up. I suspect that much of the formal literature about semps is not in English. Lynn has compiled a list of informative links regarding semps. One such link refers to the Sempervivum Journal, which is a good place to start to learn about history and has a more scientific approach.
I got a hands on demo today and learned a lot! Lynn's a fantastic teacher! 
the webbing sure makes it so cute.
