I think I've had menopause late 30s but not sure.

[Bluewavescrashing]

Hot flushes especially at night. No periods for a year. But how do you actually know.

Get blood tests

What are those self test kits like? Any good?

Go to GP, due to your age you should be on HRT to stop osteoporosis.

It can be confirmed by blood tests which check your hormone levels. This happened to my sister - menopause at 39. You need to speak to your GP as a premature menopause can put you at greater risk of menopause related health issues, so you may need HRT or other interventions.