How to tell Fake from Real  

Hmmm...
> Any sure fire ways to know before purchasing?
Well, the sure-fire way would be to have a manifest showing the chain of ownership, and the original purchase reciept.

Where is the value?
It's an interesting debate as to where the value is held when the "copy" is of an equal (and occasionally, higher) qulaity to the "original" ...

When copies are superior to the original...
That raises an interesting question: Are there cases where a reproduction is superior to the original?

Corbusier LC2 and LC3 sofas and chairs
...Immediately spring to mind. There are many "knockoffs" of better construction and materials than the Cassina authorized versions...

No no no....
Okay... I understand that there are many questions about repros vs reissues. My question is more about... how does the average person (without being a seasoned collector) figure out if the lounge chair in the flea market (or on Ebay) is an actual Eames chair, or a plycraft, or one of the myriad other copies. Did ALL the Herman Miller lounge chairs have the five star base? Were ALL the HM chairs upholstered in leather? was pleather ever offered? These are the kinds of tips I'm interested in. (and if there are resources I'd love to find them)
Whether a repro is better constructed than the original is an entirely different question.

Eames repros
Plycraft lounge chairs are definitely NOT superior in quality to the Eames Lounge Chair.

a simple answer
know your shit. sorry to be crass, but do your research and know every detail. that's how most people get good deals. know what an item is when others don't. and the only way to do it is to know. look at as many detailed photographs and read about your item of interest. there is no such thing as an authenticity radar.

^^
Crass or not, a good...
^^
Crass or not, a good answer.
I for one never spend more than five hundred dollars for anything I see online, no matter how sure I am it's authentic... the bottom line is, unless you're looking at the piece in person, documents can be faked, closeup photos of labels can be from different pieces... it's a crap shoot.
As far as buying in person goes, know your stuff- that simple.
As far as quality goes, are you concerned with beautiful design and durbaility/usability or beautiful design and a label? If I wanted, say, an Eames lounger and I found an Authentic in terrible shape for a thousand and a high-end knock-off in like-new condition for 500... I'd go with the knock-off.

Sounds like
nobody (so far) is able to specify a few definite details that would identify an Eames lounger from "the competition." I'm no expert; isn't the five-star base always found on the HM product (and may be on some copies as well) ? What about upholstery materials ? Any reliable labelling on the HM ?

whitespike is right
there is no such thing as authenticity radar; your own judgment counts. It might take a while to be a collector, but as i am concerned 40% of what i buy is mentionned vintage authentic but infact happens to be knockoffs or reissues or in the manner of. Do not worry that much about it, even auction certified items can be
sold as original but are just copies.
One other thing, if you want to collect true vintage items you have to combine both the detective and the hunter's skills.
The only difference is, that when white siberian tigers will be totally extinguished, you won't go hunting fake ones, this makes a difference with design items: billions of them are design chimeres. Good luck.

Eames vs. Plycraft & Selig So far I've bought four Eames lounge chair knockoffs at auction, labeled either Plycraft or Selig or not labeled at all. I've also seen the real thing in shops. The differences are easy to spot. The knockoffs all have screws and bolts showing on the outside. The armrests are either a single horizontal platform or they have the curved shape of the Eames armrests but are upholstered by just wrapping the leather around to the underside (no welt and no separate piece around the edge). I think the ottoman is smaller too, but the other differences are so easy to spot that I have never bothered to compare dimensions on the ottomans. I don't even look at the bases, either, though there are differences there. I've looked at a lot of other knockoffs of various iconic pieces and I've found that the differences are mostly very obvious once you know what to look for. A leg will have a different taper, hardware will show, proportions will be different, upholstery details will differ. That Hans Wegner folding chair with the woven rattan seat and back has been widely copied. Some were made in Yugoslavia and most done with fiber rush or cord (not rattan) and all with slightly different shaping of the edges of the frames and the handles on the seat. The wood is rarely as nice Wegner's and the finish is usually a glossy lacquer or varnish, not oil. Just look carefully and pay attention and pretty soon you'll know the real thing well enough to spot the fakes. That is a Plycraft chair with the cheap platform armrests. I reupholstered the chair but didn't change their design. Notice the kind of cheese base, too. And visible screws and bolts on the sides. Next is a Selig chair that I have since reupholstered. It has a heavier base that looks nicer but still has the exposed bolts. Note the really cheezy looking armrests! I redid them in the style of the Eames chair.

and the award goes to..SPANKY!
Thanks so much spanky. I've noticed some of the bolts on some chair arms. I figured some of them might amount to poor repair jobs. The edging around the armrests is a great help, as are the different base styles. With SO MANY Eames imitators out there its hard to know what you're looking at...especially on the web. I've requested the lounge chair book from my library but they havent obtained a copy yet. In the meantime I've been going back over old posts here.
It is very helpful and important to accurately know what you're dealing with. As example - a Plycraft was listed on a local message board as an Eames. When it was pointed out to the seller he dramatically dropped the price. (ethics are not dead)
When I first got into modern design I had NO idea which way was up - the collective experience of this community/message board far supersedes the knowledge I will get just reading books and websites, and I'm really grateful for that.

Ikea Eames?
I found this old article about Eames knock-offs. It says there was an Ikea version of the LCW/DCW. Has anyone seen this chair? Pictures?
http://www.citypaper.net/articles/2004-02-26/cover7.shtml

i may be wrong but
it seems you don't have as much to worry about when buying vintage knock-offs. so many are so far from original looking it is easy to decifer. i have never personally seen a vintage knock-off that looks even close. the new ones are what's scary. i know my stuff, but i found a copy in new orleans that was manufactured in the last 10 years. i had to ask the store owner if it was herman miller.