"Other posters have given you reasonable suggestions for identifying
the
cables. But I'm wondering why it matters as long as each outlet is
feeding
the same signal from the same source -- the splitter. "

Interesting you should ask (and, btw, you seem to be the first to do
so...). Anyway, a little off topic, but the reason for the question is
this: the cable company's feed starts from the attic and is being split
into various rooms as I described. What I would like to do is intall in
the attic an HTDV over-the-air antenna and feed the signal from that to
an HDTV in one of the rooms.

I don't want to run a new cable from the attic thru the second floor to
the first floor (where the HDTV is located); I just don't have the
skills (and probably tools) required for that kind of job. So, in my
infinite wisdom, I figured that if I can determine which cable goes to
the room in question, I'll just disconnect it from the splitter and
connect it to the OTA HDTV antenna in the attic (using a coax coupler)
and to the HDTV in the room and voila! OTA HDTV reception.

I understand that by doing that I give up regular cable reception in
that room but (1) it's a problem for another day and (2) it's not
really much of a problem anyway since I don't intend to user that for
the usual cable fare.

So that's the reason....