adjectives ending in -ical or in -ic

I get confused sometimes with adjectives that can end either in -ic or in -ical or in both.

For example, I always write ascetical and my proofreader always corrects it to ascetic. I get confused between energetic and energetical, though I now kind of understand the difference in meaning between the two.

I think that what confuses me the most is that though some adjectives do end in -ic, the adverb that is formed from them ends in -ically

specific – specifically, basic -basically

Is there a rule, or one just has to check every word in the dictionary?