From "Antiques Roadshow Primer"
book a little tidbit to share.
Here are some tips from that book. 
There are distintions between
"period" and "antique" furniture.

Antique--furniture that was made
 at least 100 years ago,
and though it certainly has been
"used", it does not fall into the
category of used furniture, a term
that describes second hand furniture
 less than 100 years old. A piece of
furniture can actually be antique,
however and still be less than its
 modern counterpart, because
 there were mediocre craftsmen
working a century ago, just as there
today.

Period--furniture is the plum of
the pie, the pinnacle of the antiques
pyramid. It is furniture that was made
 in the period when its design was first
popular and new, and it is generally
the most valuable of antique furniture.
The term "period" is commonly used
to refer to furniture made before the
Industrial Era or pre 1840.

The following suggestions per the book
 have this written prior...
"keep in mind that you may be dealing
with antique and valuable furniture, and
 proceed with caution"

*  Run your fingers underneath or over
 the backof the piece: very sharp edges
 and corners can indicate recent
manufacture.

* If the muslin dustcover is not original,
carefully tear back one of its corners from
 beneath an upholstered chair, and use
your fingers (a flashlight is helpful too)
to feel the blocks of wood glued in the
corners and the undersides of the
chair rails.
Edges and backs should feel hand
finished, slightly irregular and uneven
 to the touch, rounded and soft, and
never razor-sharp.

* If the upholstery is not original, lift
one edge
 at the chair rail to check for the
innumerable nail holes which, to
a trained eye, are the reassuring sign
of many reupolsterings over the course
of a long life.

* Look for the distinctive curved pattern
 left in the sawn wood by the teeth of
a circular saw. It is one important sign
 of manufacture after 1840.

* Remove one screw in some inconspiculous
spot. An old, handmade screw will
have irregular widths between the spirals,
running the whole length or the shaft.
The slot in the head may be of center.
(New screws have sharp points and
regular, evenly spaced threads.)

* When scratched lightly with a fingernail,
old pine  and poplar wood (look for it
behind chests of draws and beneath
drawer bottoms) will show a light line
on the age-darkened surface. Aged
poplar has a greenish tinge.

* Wood veneers on antique furniture are
of thick and somewhat irregular
widths, rather like home-sliced bread.
(You can judge this by noting the veneer
edges on the backs of chest tops, for
 instance, or wherever bits of veneer
have broken away.)
Modern veneers are thin, with every
slice exactly the same width.

*Check for wear on the top rails of
chairs (where dust cloths have smoothed
 away the original edge) and under the
runners of drawers, which will be
very smooth. Dining table legs maybe
 scuffed and nicked from generations
of restles feet. Some wear is good, of
course, and worn, though not
worn-out, is perfect.

* Old tables don't have metal or plastic
 taps on the bottom of their feet. (Sometimes
these have been recently added to an
old piece. Usually they haven't.)

* Turn over a tripod table. Period table
pedestals are often reinforced with a
hand hammered metal disk or a tri-part
metal strap where the legs join.
Twentieth century tables may be
reinforced with large, crimped staples.
A one-board top, made from a single
piece of wood, is a good indication of age.

*See if the piece is branded with the name
 of a manufacture. Look for a pencil,
chalk, or ink inscription on the underside
of a table or drawer. A name of any kind,
appearing antwhere on an antique piece,
is always interesting and worthy of research.
 (Stenciled numbers, however, are late-
nineteenth- to early twentieth-century
factory inventory numbers.

*Until 1800, all the mirror glass in America
 was imported. Antique glass is thin
(less than 1/8 inch thick), variably wavy,
and somewhat grey in color. To determine
 if a piece of mirror is old, hold the tip of a
 key to the glass. The closer the tip of the
refected image is to the tip of
the actual key, the more likely that the
glass is old.
                                        

*Don't be misled by where you've found
an old piece. Used furniture is as easy to
 carry up to the attic as antiques are, and
gets equally dusty. On the other hand,
valuable antique desks have been found
in laundry rooms,being used as ironing
 boards and hidden beneath layers of
scorched sheets.