Some presuppositions:
Let's say a person drinks too much for years.
'Too much' means enough to produce unacceptable consequences.
Unacceptable consequences are consequences that are unacceptable to the person.
Solution 1: moderate to two units per day, never more, and be satisfied with the outcome.
Solution 2: simply refrain from ever drinking and be satisfied with the outcome.
(If Solutions 1 or 2 are possible for a while but the result is tense misery, the scheme will fail.)
How do I know I am powerless over alcohol?
Power is the ability to do something, in this case apply Solutions 1 or 2.
Powerful over alcohol = can moderate OR can stop.
Powerless over alcohol = cannot moderate PLUS cannot stop.
For power to be exercised, two factors must be in place:
(1) Consistent willingness.
(2) The consistent ability to implement that will.
A deficit in either represents powerlessness.
How do I test whether this is the case?
Have you tried Solutions 1 and 2?
Yes: If you tried and failed, you are powerless over alcohol, because you lack the willingness or ability.
No: Try. If you try and succeed, problem solved. Bye! If you try and fail, you are powerless over alcohol, because you lack the willingness or ability.
If you've never tried and are (rightly) too scared to try, because, being presently sober, failure could spell doom:
A failure to even try, despite years of unacceptable consequences, is powerlessness at the level of willingness. Someone who does not want to do something is in the same position practically as someone who wants to but cannot. You are powerless over alcohol.
Sometimes people say, 'but I want(ed) to drink like that'. If, in an overall assessment, the consequences were or are unacceptable, the 'wanting' is not wanting in the ordinary sense of expressing the individual's overall will, but some combination of momentary impulse and insane rationalisation. Such a person is no less subject to alcoholism's mental and physical compulsions than someone having the first and subsequent drinks against their expressed will. He is equally powerless. His powerlessness is merely sugarcoated with the delusion of dominion ('I'm doing this because I want to!'), likely to allay feelings of guilt, conflict, or terror. If you don't want the consequences, you don't really want the cause, whatever the feeling of the moment, whatever the argument presented.
To drink against your will means your mind is broken: you are powerless because you lack the ability and cannot generate that ability yourself.
To drink willingly but against your best interests means your mind is broken: you are powerless because you lack the willingness and cannot generate that willingness yourself.
Whether you are powerless because you cannot summon consistent willingness or you are powerless because you cannot consistently implement that will, the problem is the same: powerlessness.
Powerlessness entails unmanageability
To manage one's life is to 'take charge of, control, or direct' it (Oxford English Dictionary, sense 6a).
If I'm powerless to avoid drinking, and powerless over the amount I drink, I am not directing my actions: the impulses to drink and continue drinking are directing my actions. My life is therefore unmanageable.