Study of books and journal articles; that is, philatelic research. There may be a philatelic library near you.
Study of your stamps.
Reading past threads on SCF.
Watching videos at
www.stamps.org or collecting societies.
Going to seminars.
These are broad topics that aren't amenable to instant answers. You need to invest time in educating yourself. You can't always expect to post a question here and get someone else to do the work for you. Sometimes you can, but not always, and the responses may be wrong.
Regarding paper, if you are talking about hard and soft papers of the Banknote era--that's a guess, because you don't say--there are many threads on SCF about this that illustrate various tests.
As to grills, roughly speaking, the number of points wide and tall are important, as is the direction of the points. You could start by measuring point ocua stamp and posting high-res scans, with your opinion, to see if others agree.