Spotting a fake is one of the hardest things to learn in the antiques trade. Only experience and knowledge will see you through this minefield. If you're not sure, the best thing to do is consult or buy with a reputable dealer.
Here are some basic tips:
Always know your product.
Study the item and look for obvious signs of ageing.
Read descriptions closely. If the description reads "in the style of" or "after", the item is a reproduction, albeit an antique.
Consider the material, the craftsmanship and style variations to try and spot whether an object is a fake.
Look at the price and think about why they might be offering it to you at such a good price... is it a bargain or just a fake?
Get a written invoice.
If the value of an antique is high or very popular with collectors, there will always be someone out there who will try to copy originals so they can profit from someone's lack of expertise.
This is not a 20th century invention. Even in the18th century, copies were being produced, so most importantly 'buyer beware', you need to do just as much research and be on the look out with antiques as with modern collectibles.
Some pottery makers will strike out their factory mark in some way. That would mean it is a factory second and will be priced a lot less than a perfect example.
It may be worthwhile visiting a shop that actually sells reproduction furniture to familiarize yourself with what these pieces look like. You could also try talking to the sales-person to glean a few tips on what to avoid when it comes to fakes.
The large number of fakes on the market makes it important to have a trained eye and a dealer you can trust. A good dealer will be able to correctly date and authenticate a piece based on a number of factors. He can tell if a piece is hand-plained, hand-doweled and hand-dovetailed. To determine periods, a piece will be judged on it's proportions, which woods would be typically used as well as the amount of shrinkage and patina the wood should exhibit.
Examine for signs of genuine wear and patina of age.
Try to determine the method of manufacturing used to create the item, and consider whether this is consistent with the piece's alleged age.
Consider the type of wood used; if the color of one part looks very different, it may be a replacement.
Are there signs of repair or refinishing?
Think about how it smells; does the interior of the chest of drawers you're looking at smell old?
Wood shrinkage is a sign of authenticity
Look for authenticating watermarks.
Since 1865, when it was first invented by Alexander Parkes, celluloid has been used as an excellent ivory substitute. Keep an eye out for these "faux ivories" names: 'celluloid', 'casein', 'French Ivory', 'Ivoride' or 'Ivorine'. With fake ivory, the grain patterns are added, so look out for very regular patterns as genuine ivory is more irregular. Spotting fake ivory is often extremely tricky, sometimes a chemical test is required to make the differential.
Silver is a great investment and it is very hard to fake.
Besides the silver stamp 925, there is a very simple way to tell if something is genuine silver; by smelling it! Try smelling an item that you know is silver and then you'll get the idea.