I am a honest man, amn't I? or
I am a honest man, aren't I?
You can also use am I not?
***** NOT A TEACHER *****
I am an honest man, aren't I?
I am an honest man, am I not?
"Bad: English (used by some native speakers):
I am an honest man, ain't I?
("Educated" people use "ain't" only when they are trying to be
humorous. Most -- NOT all -- people are embarrassed to use
"ain't." I have no doubt that a person who regularly uses "ain't"
would never be elected to high political office in this country.
We once had a president named Wilson. He reportedly said (in private)
such things as "She don't like him." But he always said "She
doesn't like him" in public. As a non-teacher and a fellow
student of English like you, I respectfully suggest that you
not use "ain't" -- except when you are in a humorous mood.)
P.S. The scholar George O. Curme (writing in the 1930's) says
that the spelling amn't is sometimes used in Ireland in the first
Amn't I after telling you she's a great help to her mother?
(Lennox Robinson, The Whiteheaded Boy)
Interestingly, 'aren't' is not particularly odd, in the spoken version. Other auxiliary verbs change the vowel in the contracted version - do/don't, will/won't, and two have the same change as am - shall/shan't, can/can't.