How can we know if a sin is intrinsically evil?

[St. Columba]

A sin is commonly called intrinsically evil if it is always sinful in every circumstance.  And what are sins that are always wrong regardless of situation?  Why, those are intrinsically evil acts.  Circular.

Is there an independent way to determine if an action is intrisically evil, absent the Church simply telling us?

I would have thought that getting drunk was intrinsically evil.  But apparently in the manuals there is provision for getting drunk when, say, a person has to undergo an extremely painful surgery (amputation, say), and if the only way to dull the pain is to drink away, then it is deemed acceptable.

[Daniel]

I think the idea is to figure out the thing's natural end and then act accordingly.

Drunkenness is always sinful, because man is a rational animal, but, if he's drunk, then he's just an animal. But maybe there are times in which a man is allowed to get drunk due to the so-called "principle of double effect"? The idea would be something like: Alcohol has two effects: it dulls the senses and it also dulls the intellect. It's sinful to dull the intellect (because man's end is intellectual), but it's not sinful to dull the senses (because man's end is not sensual). So in certain cases you may drink the alcohol, even to the point of drunkenness, if you desire only to dull your senses and you don't desire to get drunk. Even though you know you will certainly get drunk in the process.
(Note: I'm not necessarily endorsing or defending the principle of double effect. But I think maybe that's what's going on in the example you gave.)

[St. Columba]

Thanks Daniel.  But, to even possibly invoke the principle of double effect, one first has to know that the action, in itself, is not intrinsically evil. 

Killing is not intrinsically evil, neither is divorce or polygamy. Pretty serious sins for a Christian. 

But then, the Catechism (1753) says, in passing, that calumny is intrinsically evil, but the Catholic encyclopedia outlines cases where detraction/calumny can be justified!