I recently purchased an older Chopard Happy Diamond Watch from a refutable eBay dealer and wanted to share how I was able to authentic the watch was a real Chopard. 
1.      I had my jeweler tested the diamonds to insure they were real. Most jewelers have a diamond tester.  Most replica watches use crystals and cubic zirconias.  
2.      The jeweler acid tested the watch and buckle to insure it was 18k gold. (This is not a full proof test unless you scratch the surface of the watch and acid test.)  Since I did not want the scratch the surface, the jeweler only performed a surface acid test.
3.      The jeweler also removed the back of the watch to verify the movement was Chopard.
4.      Even with these tests, the jeweler could not completely verify it was a Chopard since they do not sell this caliper of watch.
5.      Replica watches do not have the same quality controls in their manufacturing process so often there are notable flaws.  Using a jeweler’s loop, check the complete watch to check for minor defect for example:
*         Hair or smudges in the crystals.
*         The face of the watch has slightly fuzzy or graphically inferior writing including letters, numbers or minute/hour markings.
*         The edges of the hands are not crisp or look inferior.  
*         Look carefully how the watch is put together. Replica watches will use inferior parts on the bands and watch.  
*         A cheap looking watch band might be a giveaway.  (Note this is not always a good indicator. The Chopard watch I purchased had an inferior leather band but the watch was authentic.)  
I recommend going through the process first before the next process. Check directly with the manufacturer.   I found the cooperate website on the internet and contacted the manufacture. The staff was able to direct me to the correct area to confirm the watch was real.  Also for a fee some manufacturers will let you send the watch to their corporate headquarters and they will verify if the watch is authentic. 
Only purchase your watch from a refutable eBay dealer. Insure they have excellent feedback and show several sharp and detailed photographs of the actual watch you are going to purchase.  For added protections, I suggest you use Paypal for your purchase.     
Research your watch on the internet; I was surprised to see large variances in prices among the different sites. This research also helped me gage the amount I should pay for a watch on eBay.
 There are several good dealers (and power sellers) that sell authentic watches. Good Luck.