How do I know if I am I going through premature menopause?

No period by age 40. If you have not had a period for 1 year before age 40 it is considered premature menopause. There are no reliable tests to tell until the periods have stopped for quite a while.
You have to wait. If menopause occurs before age 40 it is called premature. Unfortunately, blood test are not an accurate predictor of menopause until it has already gone pretty far. You can skip a period or even periods and the tests can be suggestive of menopause and then the periods can resume for months or even years.