I dont think there is a wrote in stone process for that. each person is different....what makes a person who they are is a combination of things...what they like, what they do, how they present their self....

a person who teaches children is said to be a teacher.
a funny person is said to be a comedian.
a person who cleans out and fixes household pipes is said to be a plumber.

now you take this idea and put in terms of a person who has many insiders..
what the alters do, what their job, purpose reason for being is, is what and who they are in life.

Rainy's job was to cry so who she was in life was the crier.
Red's job was to the embodiment of anger so who she was in life was the one that got angry.

when my alters integrated we all became one so now it is me the host that cries sometimes and gets angry sometimes.

who am I in life well for example I now have two children. that makes me a mom. I am married to a wonderful woman. that makes me a wife.

if you are an alter....to find out who you are just think about what you like to do, what your job /purpose/reason for being is.

if you are the host...the way to discover who you are in life is think about what you do...what kind of job you have, whether you have children or not, what your likes and dislikes are....