Counterfeit miniatures are miniatures that are illegal copies of copyrighted design. What constitutes a copy may differ from legislation to legislation - and how the producers treat it, might differ from that completely, too. This article is meant to help you recognize copies by giving examples and educating you on the techniques used, so you can make better decisions.
The basic assumption when talking about all these techniques is that the counterfeiter is looking to make some money. So the laws of economics apply: Reproduction techniques have to be cheap, done with a small workshop and must be reproducible. No counterfeiter will do a bronze cast using cire perdu techniques, for example.
What materials can be used to reproduce miniatures?
Some metals, ceramics and resin.
Modern plastics for injection-moulding are out of question - even GW does not do that on all models, because it only makes sense for really big production runs. One might argue that there are counterfeits in plastic in other niche hobbies: Airsoft players might know the fight between Chinese and Japanese companies that produce nearly the same models (KJW, KWA, KSC...). This mostly stems from them using the same moulds - Japanese companies having their stuff produced in China, then adding their own QA (hopefully...). The Chinese companies then use the moulds themselves. Since GW does not produce in China, this is pretty improbable to happen to GW miniatures. Building tools for injection-moulding is very expensive, so it is improbable that this technique would be used for producing counterfeit miniatures.
For metals, you would normally use tin-based alloys, because brass and silver, while pretty good stuff for the job, are either to expensive or require temperatures that are not reached within small workshops. Also, these temperatures break most moulds.
As for tin-based alloys, the composition of the alloy is usually a good hint. If you have old Rackham miniatures and "white metal" GW miniatures, you'll see the difference. Confrontation had a much higher detail level, because they used a softer alloy. A reproduction will usually use a different alloy that in most cases produces a worse result that the original material. A loss of detail usually is the result. This is the main indicator for counterfeit miniatures made from tin.
Lead might also be used. Even older commercially made miniatures were made of lead, until it was found to be a health risk. Lead can be treated like tin.
Resin is another candidate for re-casts. Resin is lighter than tin.
Ceramics may also be used. Ceramics are much lighter than tin or even plastics.
Also, the choice of material can give away a counterfeit miniature. For example: If you find the modern-style terminators (the bigger ones) made from tin, they are a forgery, because they were only made in plastic. This requires a bit of knowledge on the subject, though: The current (as of 2/2009) Chaos terminators are made of plastic, yet they are very similar to their predecessors made of tin.
How would one build moulds?
Modern moulds used by GW have a pretty slick design. If you follow the mould lines, few of them are made with two rectangular blocks as mould halves. Mould lines usually go through the most unobtrusive parts, like hair instead of the face of a miniature. Mimicking this is possible using rubber moulds, but it is hard to do. A hint therefore would be the placement of mould lines.
Additional mould lines might be results of a re-cast of a miniature that was not properly cleaned. So mould lines that are not necessary, because they are redundant also indicate a counterfeit miniature.
Moulds also restrict the available materials a lot. You cannot cast brass in rubber moulds, unless you use really expensive moulding material. Economic considerations therefore should be taken: Re-casting stuff to sell it only makes sense if you can reuse the form often. Rubber moulds will last a hobbyist for quite a while, but producing enough to sell is not easily done with the finer moulds. Again, this would lead to a loss of detail: Edges get duller and parts of the mould simply break off. Please note: Old tin Rackham miniatures also showed the wear and tear of moulds reused too often, so you should know the average quality of a line of miniatures before you decide whether it is counterfeit or just badly made.
How do you use moulds?
Creating moulds is a topic you learn for quite some time as an apprentice machinist, so it is not that easy. There are a lot of mistakes to make and to recognize:
One typical problem is making the canal for the metal too short. You need some weight pressing the metal in the lowest parts of the form, so a mould has to have a big cone on the top where you pour the metal. And you need it to be very long, so the metal can cool down properly above the miniature. Making it not big enough leads to improper casting on the extremities of the miniature. Making it too short means a lot of heat on the upper part. Both result in a loss of details in those places.
Since fitting forms sometimes is a problem (putting pressure on the rubber mould to make it fit snugly), mould lines tend to extend a lot, creating small areas of very thin material, also called "flash". This can be seen on FW resin casts, too, but on tin (commercially made in very sturdy and expensive moulds), this is rare.
Selling counterfeit miniatures
While there might be some counterfeit miniatures made by people dealing with this casting, like dentists, machinists and goldsmiths, even those miniatures won't come in original boxes. Proper casting might be expensive, but proper offset-printing of cardboard is even more expensive for small runs. So counterfeit miniatures are usually sold without boxes.
Used might be either blank or primed or even painted.
Priming is used to obscure the colour of the original miniature, because this might give the choice of material away. A primed ceramic miniature with a heavier base might even weigh as much as a tin one. A thinly primed miniature with rather blunt edges also might be though of as an original with a coating that is too thick.
Painting may simply be used as priming, but also to hide the lack of details. Every detailing technique may be used to distract from blunted details.
Also, the type of offer might give away that it is counterfeit: While copying one miniature properly is certainly possible, it is usually not feasible to copy a whole box of different models. So a set of several identical models might be a hint.
Good candidates for counterfeits are either limited miniatures, hero miniatures or those that have not many varieties, especially, if they are expensive: 40k obliterators and wraithguard come to mind.
Since loss of detail has been mentioned several times, it shows another class of candidates for counterfeits: Models with few details. This might mean old armorcast models or models with large, empty spaces like wraithlords.