I'm not sure exactly what you mean but here's what
I know.

If you mean 'trimmed' as in some owner down the
line cut the cel for framing/display purposes or
easy storage, that can happen with normal sized
cels too.   There's no absolute way to tell. 
However some signs may be that other cels in the
sequence are a different size or that the paint
goes all the way to the suspected cut edge.  Be
careful though, these don't necessarily mean it
was cut.  If a previous owner tells you it was
cut, you can ask them how they know.

Depending on how much was trimmed and whether the
buyer knows/can tell, the market value of the cel
may decline.  In some cases people may not care. 
I personally dislike my cels tampered with after
the fact, but if it's not too severe I may let it
slide.  Personally, I would count it as a factor
against the cel.  I believe someone once mentioned
an episode of Antiques Roadshow where one woman's
collection of Disney was considered "lesser"
because she had cut all the cels down to just the
character-painted portion of the acetate.

I have come across backgrounds which are stamped
by the studio/company which made them.  I have
come across backgrounds that have some writing on
them too.  I have -not- come across a background
that had someone's signature on it.  It may be
someone's fan-done background (kudos to the artist
for signing if that's the case), but it's again
difficult to make any sweeping comments.

Keys