I have the unfortunate honor, of presenting first hand information on the subject with everyone. While searching for a good deal on fountain pens in Ebay, I came across a Parker Sonnet at a too-good-to-be-true price. Despite noticing that the seller is from China, I threw caution, and common sense, into the wind, bid for the pen, and won.
Went the pen arrived, it felt wrong right from the start. The weight of the pen was lighter than the rest of my Sonnets, and the material felt a little cheap. A closer look revealed workmanship errors which was uncharacteristic of the Parker Sonnets.
Here are some comparison photos to help fellow enthusiasts in identifying counterfeit Sonnets.
"Jewel" on top of the cap
While the quality of lower end Parkers have been questionable these past few years, they have still maintained some standards in their mid and higher-end products. The plastic "Jewel" on the top of the Sonnet cap should appear center, as shown on the left, and not touching the edge like the counterfeit on the right.
The depth of the lines on the arrow clip
One of the things that eluded Parker counterfeiters, has been the "feather" lines on the Parker arrow clips. Most of the time, the lines will be too shallow or not clear enough on a counterfeit (as shown on the right), though this could be hard to detect without a comparison.
Length of the threads
Small details, like thread count, isn't something most counterfeiters pay too much attention about. As shown on the right, this example of a counterfeit has a shorter thread than standard Sonnets, I have seen examples with longer thread counts in other Parker products.
The thread count of feed to section is about 1 and a half turns on a standard Sonnet, but this counterfeit requires 3 full turns.
This is the first test which I use for any Parker pens which I have doubts on. Modern Parker nibs are made of a non-steel alloy and should not be attracted by a magnet. As you can see on the right, the "18k" nib is obviously not what it claims to be.
Do note that counterfeiters have caught on recently, and I have a counterfeit of a Parker IM which can pass this test (failed the previous test though).
For the Sonnet, the nib of the pen is fastened onto the feed mechanically, and should be a secure, non-moving fit. This example of a counterfeit has a feed that moves away from the feed on slight pressure.
I have already raised a dispute on paypal for this counterfeit, and even if I don't get my money back, the price I paid for it is low enough to be an inexpensive "lesson fee". Good hunting!