Acquire an initial feel and ideas by surfing the Web.
Visit museums and auctions while travelling overseas. These are great places to look around and study individual pieces of renowned jewellery.
By looking at the pieces, you can make your own price estimates and then compare them with the actual prices the pieces have been sold for.
Visit well-established antique and vintage jewellers.
One important name is Cartier, noted for its outstanding artistic achievements. The most desirable period is from 1900 to the 1960s. There were three separate Cartier operations - in Paris, London and New York - with three distinct styles run by the three sons of Alfred Cartier. The one in Paris specialised in opulent designs featuring a lot of coloured gemstones; the New York operation was known for its avant-garde designs; and the one in London produced jewellery of more traditional, classic English style.
Other great names include Van Cleef & Arpels, the inventor of the invisible setting and known for its superb craftsmanship. The desirable period is from the 1920s to the 1970s.