-tion, and -sion

Hm it's a tough one. One thing native speakers seem to do with that is play with mispronouncing the word to remember irregular spelling.

So with "mention", of course remember the normal pronunciation "men-shun", but also play with an exaggerated version highlighting the correct spelling, like "ment-un".

I'm not sure if that's helpful advice or not ^^

生憎ですが、綴りを暗記する方法しかないと思います。発音はほぼ同じなので、綴りの苦手なアメリカ人にもかなり難しいではないかと思いますが。

When "sion" and "tion" come after an 'n', they do sound exactly the same; so unfortunately you will simply have to memorize the spellings in that case. Fortunately, the number of such words is not large. Here are a few:

tension, mansion
mention

When they come after a vowel, the pronunciations are actually different. "Tion" is pronounced with an unvoiced "sh" sound, whereas "sion" is pronounced with the corresponding voiced sound, which doesn't exist in Japanese. You will probably find it hard to hear the difference at first, but you will need to listen to some of these words in order to learn how to pronounce them properly, anyway. Once you can hear the difference, the pronunciation of the word will tell you how to spell it in almost all cases. I recommend listening to some of these words with a pronouncing dictionary:

explosion, evasion, erosion
pronunciation, motion, edition, vacation

After a consonant other than 'n', it is almost always "tion" (I can't think of any exceptions, offhand):

action, caption, fiction, friction, eruption, option

There are some cases in which the "tion" sound after a vowel is spelled "ssion" instead:

passion, mission, fission, emission

Then there's the word "fashion", which rhymes with "passion".

As with most other spelling issues in English, there is no simple rule which works all the time. I hope these observations help a tiny bit.

Actually, in my opinion the pronunciation is slightly different. The -sion sound is usually a bit softer than -tion.

After most consonants, -tion sounds like "ch" while -sion sounds more like "sh". For example:

mention = "menchun"
tension = "tenshun"
question = "queschun"
mission = "mishun"

After vowels and the letter "r", -tion usually sounds like "sh" while -sion sounds more like "zh". For example:

ignition = "ignishun"
vision = "vizhun"
nation = "nashun"
invasion = "invazhun"