If you suspect you are going through POF/POI/early menopause, the first step is to get these hormone levels tested:

Follicle Stimulating Hormone (FSH), Luteinising Hormone (LH) and oestradiol (E2 oestrogen) 
These tests should be done on the third day of your cycle (if you still have one) and should be done at least twice as hormone levels can and do fluctuate naturally. High FSH, high LH and low E2 is indicative of POF/POI.
If your FSH level is above 10 this would require discussion and possible referral to a specialist (endocrinologist or gynaecologist). 

If these tests confirm you/your doctor’s suspicions, here are some further tests for consideration:

Baseline bone density or DEXA scan 
The need for a repeat scan will be guided by result and decisions around oestrogen replacement. Don’t worry, this is very low radiation compared to normal X-Rays.

Blood tests
Iron levels, vitamin D, glucose, lipids, cortisol, thyroid and antibodies – adrenal, antiendomysial, thyroid, testosterone.

If you find a worrying lump in your breast, an ultrasound scan is better than mammography for viewing younger (denser) breast tissue – and far less painful!

This is a general guide not a medical opinion and should not replace a consultation with your healthcare professional. Everyone has a unique medical background and your doctor should discuss with you the tests required and what your test results mean for you.