n8xct,
Well, wmlovell gave you the "Wiki" reference; however, it is mostly overlooked, severely outdated and, generally just not kept current with information when it comes to what you are asking for....not his fault; just the nature of how Wiki's are around here...
phask gave a reference for the "ongoing" information stream about the system; however, it generally morphs into a large, cumbersome, sometimes helpful line of info....
S-o-o-o-o.... for what you are asking, there is no really good way to answer who you may be "hearing/seeing" on your radio unless you either ask on here (as the sticky MARCS-IP topic is for that).
.... or, set out do some research of your own; like I have been doing with Unitrunker, all the while, compiling a list of unknowns for my area....
It would be nice if we could break down the large MARCS-IP topic into regional sections to help with "searching", users asking for help, etc..... and, because, honestly, I could really give two toots about some of the bigger metro areas that generate most of the system traffic.
I know what I just said may not help you much; but, that's part of the issue with "unknowns"..... I've got a pretty sizeable list of "unknowns" for my area in Southwest Ohio/Greater Dayton area, so, I know where you're coming from on this topic....
More than willing to try to help, if you give a shout...