How can I tell if a cancellation is fake on an 1800's stamp?

Thanks for your post, Henry (henrye ).  I would encourage you to include images with your posts, as it always gives members more to go on when trying to respond.

You did an exemplary job of that on the Bavaria gray-color 20-pfennig issue, and it really made the discussion get rolling.  If you are able to provide some examples of the stamps you are concerned about, it will give some great visual context to your question.

henrye  here is a link   to a Hamburg stamp with a cancellation similar to your first stamp (although  not the same denomination). There are a number of stamps on e-Bay, some having certificates (which may not be conclusive as fake certificates have been known of)

 I would ask for a cert. !!!!!!!!! and check the certification after

Postmarks expertise on a Web scan makes me tremble 


René

Forged postmarks on Thurn & Taxis are abundant. 

This is what I would recommend for your reference library:

A complete collection of images of genuine to compare against. The first 25:


Then, after looking at a lot of postmarks, you will notice the genuine have the following characteristics: 

• The numerals are well formed and consistent 
• The numerals are often curved with long serifs 
• The end balls on numerals such as 5, 6, 9 tend to be large 
• Serifs on the stem of 1, 7 tend to be long on both sides
• The rings tend to be even, not blotchy and evenly spaced

For the fun of it, compare "treasure" on eBay to your ref. collection:

A few examples (genuine on the left):







Down this long fun/frustrating road of research begs the question: Is your own specimen genuine? 

I would guess there is a 3rd (1st) digit, but hard to see.

This complicates things. As you can see, there are different 4's:

I have two:

Looking at the the last two images you posted is interesting.

Especially when you get the negative image.   the 424 is odd.  In the negative, it looks as if the 2 has a 'shadow'.

It makes me wonder if some of the ones that are supposed to be fake, may not be.    What if a postmaster had to recut or otherwise repair his/her cancellation stamper?   There would be little to no consistancy and we would even see some that looked downright sloppy.  Being a good postmaster was probably more important than the ability to make/alter/repair a cancellation stamper.