This was (the) one thing the hospital I was in the other day was good at - when I told them I was diabetic, they asked what medication I was on and how much, then asked if I did the injections myself (possibly they asked because of my almost hysterical reaction to the blood test needle!), when I said yes they just said OK and were satisfied with that, they did not try to take the insulin or interfere in any way.
They did regular blood tests, with a cool tester that had a barcode scanner to scan my wristband, and were happy for me to use my own lancet device - their one was evil, with no depth settings.
So not all hospitals will steal your insulin, some are more enlightened!