Start with the simple stuff
Well, you have written a bunchlist of rules and most of them are non fatal-fatal if tested, so. So you can start with the simple and "safe" stuff, and then you move down the list.
Objects are absorbed
Well I have a slight objection to that one: absorbed as in encased and kept indefinitely or dissolved and excreted?
Test: Push a sewing needle into your arm.
Success: Needle is gone after some amount of time.
Failure: Needle causes discomfort, infection, inflammation or otherwise hurt you.
Keeping safe from failure: Visit a doctor and have the needle extracted.
Cannot drown
Test: Jump into a pool at the shallow end and crouch down. Bring a buddy if you wish for extra safety.
Success: Still alive after 10 minutes with no ill effects.
Failure: Feeling the usual feelings of panic and discomfort of not being able to breathe. Starting to get grey-out and tunnel vision.
Keeping safe from failure: Stand up and breathe.
Cannot get fat
Test: Eat excessively. Monitor your body temperature
Success: You do not get fat. Body temperature rises.
Failure: You gain weight.
Keeping safe from failure: Stop over-eating.
Fast healing 1
Test: injure yourself lightly. See how long it takes to heal and if that varies depending on how much you eat.
Success: Wounds heal without scarring or discoloration, and they do so depending on your food intake.
Failure: Scarring results. Wound do not heal any faster compared to what they would normally.
Keeping safe from failure: Stop injuring yourself
Fast healing 2
Test: Cut off some part of yourself, like a centimeter wide flap of skin, as if say removing a birthmark or a mole. That cannot heal on a normal person without scarring.
Success: The cut is healed without scarring.
Failure: The cut is not healed without scarring.
Keeping safe from failure: Sorry, you will have to put up with that scar.
Fast healing 3
Test: Donate blood five days in a row. No normal person reacts well to losing five pints of blood in that short amount of time.
Success: You do not experience dizziness or other signs of low blood pressure.
Failure: You start to feel the symptoms of blood loss.
Keeping safe from failure: Stop donating blood as soon as you feel the least bit ill.
Fast healing 4
Test: Cut off the tip of a finger
Success: The finger heals without trace of injury
Failure: The fingertip does not regenerate
Keeping safe from failure: Again, sorry, it is gone... but a fingertip is easy to live without.
Fast healing 5
Test: Stab yourself in the lung
Success: You heal without sign of injury
Failure: You start experiencing great difficulty to breathe
Keeping safe from failure: Call emergency services and have them patch you up at the hospital. Blame a mugging or something like that.
Fast healing 6
I think you get the idea by now...