A GAD antibody test will tell you if your body is producing certain Antibodies that attack your pancreas. But there is some disagreement in the medical community whether everyone who tests positive will develop Type 1. You may also want to have a C Peptide test to test your insulin levels. I am one of those thin type 2's, but I know I am insulin resistant because Metformin does work for me. There are a lot of Type 2's who also have to use insulin. I do think there is a lot of crossover with symptoms of type 1's and type 2's. It really doesn't matter what the diagnosis is as long as you are using the correct insulins to bring down bgs to normal levels. As far as weight gain on insulin, that is common. As you have found Low Carb diet is one thing that will prevent that. The more carbs you store as glycogen in your body the more water you will retain. That is why initially when you go on a LC diet you lose a lot in ther first few weeks. It is all water weight.