You can donate blood, ask your doctor (insurance may not cover it), or use a simple at home test like this (Eldoncard Blood Type Test (Complete Kit) - Air Seale

You can donate blood, ask your doctor (insurance may not cover it), or use a simple at home test like this (Eldoncard Blood Type Test (Complete Kit) - Air Sealed Envelope, Safety Lancet, Micropipette, Cleansing Swab https://smile.amazon.com/dp/B00JFTSPMW/ref=cm_sw_r_cp_api_i_LT66Eb6S9KS02 https://smile.amazon.com/dp/B00JFTSPMW/ref=cm_sw_r_cp_api_i_...). I used one of the card tests and got an O- result, then when I had annual bloodwork done I asked my doctor to test it just because an O- result on the cards looks the same as a possibly defective card (card wasn’t defective, I did in fact beat the 25% odds and inherited the negative Rh factor from both my O+ parents).