Identifying Revenue Stamps Affixed To Documents

Somehow soaking a revenue stamp off of a document reduces the value in my opinion. So, how do you all identify the different types of revenue stamps when you can't see the backside?

I'm a little perplexed on this one. I bought a pile of bank checks, stock transfers, and other documents form ebay the other day dating from 1860 - 1910. So, how does one identify stamps when stuck to a document?

And please, if I'm not seeing the forest for the trees let me know.

Thanks ya'll.

Well, on the early issues the only identification you're possibly not going to be able to do when stuck to a document is 1st issue silk paper.

Watermarks don't really become an issue for differentiation until issues of 1914 (I'm ignoring the provisional overprints).

Some of the documents are partial... Some one before me tore off just the area the stamp was on. Seems that those could be soaked off.

Any tips on soaking 1st issue rev stamps?