How to know when the z can't be used instead of s in an ending? [duplicate]

I'm familiar with -se -ses -sation etc endings being British and the American equivalent being with z rather than s. However, I stumbled on the word "improvisation", which apparently can't be spelled with a z.

How can you know which words are spelled using 's' in American English?

When Noah Webster was busy compiling his dictionary in America in the 1820's he wanted to make a clear break from the British spellings, as the Revolutionary War had occurred and America was now independent from Britain. This is how we get words that omit the 'u' - flavo(u)r, colo(u)r, etc. As well as centre/centre.

And the 'sation/zation endings that you ask about.

He didn't want to change the appearance/spelling of the word that much but make it distinct as well. And in the computer age most programs are auto set to USA spellings now.

http://en.wikipedia.org/wiki/Webster%27s_Dictionary