Haunt antiques and collectibles stores as well as auctions to familiarize your eye. If you look at the best, eventually your eye will tell you when things aren't right.
Examine old items for signs of damage, which can be a tip-off as to authenticity. There's honest damage and honest dirt, and if an item has lived for 200 years, it will have scratches. But fakers will usually overdo it.
Don't put too much trust in makers' marks, because they're commonly found on fakes as well as originals. Ann Miller, a porcelain expert, says she always examines a piece carefully before turning it over to see the mark. And she asks herself: Does it look like the work of a particular manufacturer? Are the color, size and other features appropriate for that manufacturer? If something is off, she knows not to trust the mark.
Know that age doesn't automatically confer value. Know, too, that collectibles can vary greatly in worth even when they're made by the same company. For example, a particular kind of old red glassware is worth a lot, she notes, but the same pattern in blue glass isn't worth nearly as much because there's more of it available....this is where reading will help you be informed.
Get to know antique dealers. There are some really good dealers out there, and the best ones are willing to share their considerable knowledge. I've met some wonderful antique sellers here on eBay who are more than willing to be of help!
If you're buying items primarily for investment, buy the best you can afford and make sure the items are genuine and in mint condition, with no repairs or replacements. You can be more lenient if you're buying primarily for your personal enjoyment.
Finally, accept the fact that you may occasionally get taken. If you buy something for $100 and find out it's worth $50, that other $50 is the price of experience....happens to us all.
And it's not the worst thing that could happen. The terrible times are when you realize something you missed. Yes, I have let really good deals get away, I recall two carved Italian figurines I could have bought for about $70. I passed them up, and not long after they sold for $40,000. I'm STILL sick about that one!