Angus's article prompted me to investigate. Some will point to the etymology of a word to assist, but that doesn't always help. I decided to pull all the words ending in -able or -ible and see if there is an easier way to know. For me if I'm in doubt the easiest way I know is to use Word Check which is an online tool I wrote.
The dictionary tools I write also mean this is not an issue for me. The leading authoritative references such as the Macquarie and Oxford dictionaries will often list the spelling used the most, but also state other spelling variations. Not particularly helpful. The dictionary tools I write makes it easier as only the preferred Australian English spelling is considered correct and secondary variations are marked as spelling errors. Since I have these dictionary tools installed across all the applications and operating systems I use this is no longer an issue for me.
However I decided to go one step further. In reviewing the words I found there was only 65 base words which contain -ible, whereas there are significantly more words ending in -able. Thus if you take the time to learn the 65 words (many of which are obvious), you'll know if it isn't one of these words then the spelling should be -able. There are a few words with -eble, -oble, or -uble endings, but generally these words are obvious and thus not an issue either.
Many people don't appreciate the time involved in this research. I've spent the last 8-9 years working on the preferred Australian English spelling dictionary so with that I'm well placed to create a list of words. Even then I needed to crosscheck a number of words. This enabled me to find and remove one word which I'd incorrectly added with both spelling variations and add another 10 words I'd not previously included in the dictionary. Overall around two hours of work. Since very few people wish to pay for such information it is highly unlikely the time will ever be recovered.
For those who are interested, I've added a MyAnswers solution which documents the 65 base words ending in -ible which can be found at www.onlineconnections.com.au/myanswers/mapurchase.php?solution=2484.
The changes to the dictionary as a result of this investigation have now been made to the word list and will be available in the next and future releases of my dictionary work.
PS. Whilst not meaning to disparage the article written by Angus, I did notice the usage of the secondary spelling variation spelled, which should be spelt. There is also the use of the hyphen in no-one, which should be no one, without the hyphen. No one is perfect and I've certainly learnt and made a change to my work as a result of reading Angus's article. Thank you Angus.