It would be fairly safe to say you can add -able to any verb that can bear the construction "can be + past participle" (this can be said → it is sayable), or as snailboat/plane pointed out all transitive verbs.
But:
1- The suffix is not always spellable as -able. It will be spelled -ible with a few verbs whose common point is to have a Latin root. I do not know why with some and not others, it may have something to do with the way the suffixation of the word was formed in Latin.
A few examples:
- This post is perfectible.
- Grammar rules aren't really flexible.
- A collapsible bed (although I've already met a collapsable bed).
Sometimes the -able and -ible adjectives exist alongside, usually with a nuance in meaning. In these cases the -ible adjective has come directly from a Latin verb that has not made its way in present day English, and the -able adjective is formed from the present English verb of Saxon origin.
This cake is quite eatable means "it tastes nice". This cake is edible means I haven't put any poisonous substance in it.
A hearable sound (rarely used, I admit) is nice to hear, an audible sound has a physical quality (loudness for example) that makes you can hear it.
2- Sometimes the root of the verb will be slightly modified:
- verbs ending in -ate: navigable, translatable...
- (in)comprehensible. (I can't think of any other verb ending with -hend to generalize further)