-ible or -able

I was always told to guess "-able" on a debateable spelling, because there are "many more" of them. For some reason, I decided to see if there was a more reliable rule I could find on the internet.

Sort of. To be quite sure, one could look it up - not always practical in the moment - or enter the word into a document that has a spellcheck feature. But once in awhile, you still have to guess, don't have a ready source, and hope not to look sloppy. In addition to the link's info, know that "many more" in my first paragraph turns out to be about 80%. The rule at the link about whether a word comes from Latin or not strikes me as unhelpful. It is not always obvious, even to those who pay attention to such things. Plus, that's not 100% certain anyway. The rule of thumb at the bottom is sort of interesting, though.