Start with the simple stuff

Well, you have written a list of rules and most of them are non-fatal if tested. So you can start with the simple and "safe" stuff, and then move down the list.

Objects are absorbed

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...