It is a good idea to get in the habit of examining EVERY pen - top to bottom.  Remember that most vintage items were used on a regular basis. That means they may have also been repaired or serviced- professionally or otherwise. 

Just because the pen has never left "Uncle Harry's" house does NOT mean "Uncle Harry" didn't one day decide to save himself a repair bill & take the pen apart with pliers in order to figure out why it all of a sudden stopped writing OR glue on trim that loosened over the years OR use it as a paint stick OR any other crazy thing you can think of (and several that you can't).
 
Caps

Check for cracks - especially around the "lip" or edge which often occur from placing the cap over the end of the barrel while writing. Also check for warping, missing or worn trim; scratches; dents in metal; chew marks or discolorations. Color should be even & consistent & match that of the barrel (assuming the barrel is not discolored) Also check fit of threads. When twisted on the cap should be snug - not wobbly.
Trim

All metal trim - including lever & lever box should be checked for plate wear.  Make sure trim is not missing, loose or glued in place.  Also check that trims are original to the pen.
Again, some slight wear on raised areas of trim can be expected.  If the pen was used regularly it will have been in constant contact with  clothing.  However excessive wear - like that resulting from the use of buffing wheels is not acceptable.
Clips

Check for plate wear.  Also make sure it is straight, snug & has not been pried away from the cap at either end.
Traditionally, pens were/are carried in a breast pocket.  Clips are usually in constant contact with the inside of a vest or jacket so slight wear on areas that would rub is the norm.  Anything more than slight, normal wear needs to be evaluated based on YOUR standards of what is acceptable in your collection.
Barrels

Same as caps.  In addition, check for  monogramming or  initialing   - professional or "do it yourself" versions - and condition of imprint.  This is also where you will check any mechanics of the pen.
Warping - pens can become misshapen due to exposure to heat.  They can also end up with a "flat side" where someone tried to remove Uncle Harry's initial.  Sometimes this is obvious - sometimes it's not.  Roll the  barrel on a flat surface.  If it doesn't roll smoothly - take a better look. 
Levers

Levers should move freely but will sometimes appear to be stuck due to a hardened bladder.  NEVER force a lever to move- if the bladder is hard the lever will break or bend. 
If it doesn't want to budge determine if the bladder is indeed the problem. Many times levers have been glued in to the lever box to hide the fact they are broken.
Nibs

Check for cracks, bends and missing iridium or tines. Also check for alignment and fit and make sure the nib is original to the pen.  Warranted 14k nibs are usually "generic" replacements. Last, but not least, check that it writes smoothly.  The nib should not scratch  - it doesn't matter if there is ink in the pen or not.
Feeds

Again, check for correct fit to both section & nib, pry marks, cracks or signs of glue.
Sections

Check for cracks, snug fit and make sure it has not been glued in.  On many occasions, sections will show marks from pliers and/or other signs that an inexperienced person has tried to remove them.

 
The pen shown above is by Paul Wirt Fountain Pen of Bloomsburg, Pa.  It is  a NOS, tapered slip cap, eyedropper filled pen featuring an  8 sided barrel with alternating mother of pearl & abalone.  It is part of Bruce's collection & is shown here simply because I wanted a picture to break up all this text.  :-)