I found a site called piggybankpage and they have alot of useful information about the fakes that are about and how to spot them.
Here are a few pointers but the site is well worth checking out before you a place a bid on any Natwest Pig.

Wesley with a green shirt and red shorts and hat is a fake.
Wesley with a green hat and shorts with yellow shirt is a fake.
White jacketted sir Nathaniel is a fake and Wade no nothing about these.

The woodys that were made by the sunshine company would have NO markings on the bottom. The ones made by Wade will be stamped accordingly
Some of the annabels issued by the Bank do have Made in England stamped on the bottom.

Any pig made by Wade would have Wade stamped on the bottom

They tell us that the 22ct gold leaf Woody is very very rare and highly unlikely to come on the market should you stumble across one, please be aware that they were made by Wade and should be stamped accordingly. All the Gold Woodys should have their certificate of authenticity of some description as they were given to Board of Directors and another 25 given as a prize for a treasure hunt.
The sunshine company made no Gold Pigs

Please take a look at this site or at least some one elses who knows whats what before you part with your cash.