Portugal - Enamel Surfaced Paper?

I'm looking at the early years of Portugal and the Specialized Scott cat lists plain paper and enamel surfaced paper. How might one determine the difference between the two reliably?

Thank you!

You will recognize enamel-surface paper immediately when you see it. It has a very glossy porcelain-like "glazed" surface.

[EDIT: After reading your following post, I realize that "glazed" was a poor word choice. A better way to describe is a very white shell-like finish.]

I think I've discerned some of them, although I've noticed a few that have parts of the stamp glossy with the majority being unglossed. Is this a common occurrence and classified as enamel? Thank you for your reply!

Are these used stamps or mint stamps?

I don't recall this being a problem for me on the King Luiz issue, which came as plain vs enamel (disregarding reprints). You did not specify which Portuguese issue you were referring to, so now I am wondering if you are talking about the King Carlos issue, which had enamel-surfaced and chalky-paper varieties. The chalky paper will have a rather uneven sheen, that may even disappear completely in large sections for used stamps that have been soaked in water. The enamel-surfaced paper will have a very even glossy surface, almost like porcelain; the paper will appear very white with a hard shell-like finish. Like I said, it is unmistakeable. If you have any doubts, then it is either not enamel-surfaced or it is a damaged stamp. Not sure if my post is helpful or not.

If anybody else has had a different experience, please feel free to chime in. This is all taken from memory...

The stamps are all used and removed from paper (not by myself so not sure of the process). I think they are both sets, scott numbers 40's through 60's. Here is a picture posted online of a plain and enamel and these are some of the ones I am struggling with:
http://www.saaphilnet.com/photos/91084.jpg