{I sent in something on this before but I believe it got lost in the
weekend accident the moderator described.  This is an improved
version anyway so no loss the first time.  HL}

The standard work on detecting forgeries of ancient documents 
is probably the writing of Friedrich Blass, "Hermeneutrik und 
Kritik," _Einleitende und Hilfsdisziplinen_, vol 1 of
_Handbuch der Klassischen Altertums- wissenshaft_ (Noerdlingen:
Beck, 1886).  Portions of this are described in Nibley, _The 
Prophetic Book op Mormon_, pp 219-242 (SLC:  Deseret Book, 1989).
(If you want to attempt reading this be forewarned.  Nibley
describes Blass as a typical German scholar who claims little
knowledge of his subject, then proceeds to exhaust both the subject
and the reader.)  Nibley's extract from Blass's work is in the form
of "rules for forgers."  It makes interesting reading.

I confess that I have not read Blass's work, only Nibley's extract
thereof.  My German falls far short of what would be required and
as far as I know there is no English translation available.  However,
I believe the techniques he describes are known widely enough that 
any competent classical scholar could examine a purportedly ancient
document and at least determine if it is consistent with what one 
would expect of a genuine document of that time frame.  We will not 
be able to prove who wrote it but at least we should be able to 
determine with reasonable confidence if it is from that time and 
culture or is a later forgery.

Actually there are 2 types of purportedly ancient documents:

1.  Alleged actual holographs or early copies thereof.  For example
the Dead Sea Scrolls.  These can be tested by various scientific
means to determine the age of the paper, inks, and objects found
with them.  This can provide a pretty clear dating of the actual
physical objects.

2.  Documents claiming to be copies of ancient works although the
copy itself may be much more recent.  For example we might find a
document which monks in a monastary claim is a copy of something
from centuries ago (perhaps even having been through several
generations of copists).  This is more of a problem but can still 
be tested (although the test is not likely to be simple).  We cannot
expect a test of the age of the physical objects to tell us much so
we must confine our testing to the text itself.

It is important to remember that none of these tests can tell us if
the document is really what it claims to be.  They can only date the
document and identify its culture of origin.  For example I've heard
of a letter supposed to have been written by Jesus himself to a king
in what is now Iraq.  If this document were to actually turn up
scholars could date the paper and ink (assuming they have the
holograph).  They could check the language, content and writing
style to see if they are consistent with what would be expected of a
Palestinian Jew of that time.  However even if all test results were
positive there is no way to determine if Jesus himself actually
wrote it.  We would know what time and culture it came from but
(barring a known sample of Jesus handwriting or other clues for 
comparison) scholarship must stop there.  There is seldom any way to
determine who the actual author was.

As I say, I'm no expert on Blas's work.  I do remember some of the
tests which can be applied to alleged copies of ancient works.  
Specifically we might ask:

1.  Is the document internally consistent?  Does it contradict
itself?  If the work it is short it would be relatively easy to 
maintain internal consistency, even if it is a forgery.  The 
longer the forgery the more difficult it is to maintain consistency.
For this reason most successful forgers stick to short documents.

2.  Is it consistent with the history and geography of the time?
Again a short, non-specific work might not be testable but if the
writing is of any significant length no latter-day forger would be
able to escape detection.  Here we look for the minor, inconspicuous
things which someone from that culture would get right without even
thinking about it but which a later forger would find too numerous 
and trivial to check.  The devil is in the details.

3.  What about the literary style of the work, figures of speech
etc.  Any ancient writer would almost certainly speak in ways that 
seem strange to us.  Are there any such odd phrases in this book?  
If so do they fit in with the culture?

Of course there are complications if the document has been
translated, or possibly even if somebody just updated language
when he copied it.  A few cases of language not from the culture
claimed may be allowed in recent copies.  They cause problems and
reduce certainty to be sure but don't necessarily prove forgery.

These tests can be quite effective (given enough material to work 
with) but they are not easy.  They require the skills of the 
historian, the linguist, the anthropologist etc.  The questions to 
ask are, "Is every aspect of this document consistent with what we 
know about the culture of claimed origin?"  If there are things 
which don't fit how significant are they?  Are problem areas due to
our lack of knowledge, later changes by copists or are they really 
significant?  There will often be some ambiguity since we never
know everything about the culture.