It isn't easy, .
Though there are some adults that have a rapid onset of T 1 , it is more often a slower process with a gradual loss of beta cell function This results in similar symptoms to someone who is T2 and has a relative insulin deficiency. People who are diagnosed early in the process may be able to control their diabetes with oral medications or even diet and exercise for a while. Eventually though, whatever they do they will need insulin.
Tests that can help distinguish are those for a number of antibodies and C peptide which measures insulin production. In some parts of Europe people who don't fit the normal profile for T2 are routinely given antiGAD tests. (anti GAD is the most common antibody in LADA)
However, it isn't infallible Some people don't test positivive for antibodies (even some children don't). Some people who are claimed to be 'metabolically' T2 do...and these people are also likely to need insulin sooner than later.(so are they really T2?)
Whether tests are done to help diagnose whether they are type 1 or not very much depends upon where they live and who their doctor is. Many may stay diagnosed as type 2. In the UK this can limit their treatment (not eligible for either DAFNE courses or insulin pumps and more likely to be given mixed insulins rather than MDI)
I left it very late to go to a doctor. I had dropped from just over 10st which was slightly overweight to 7st 8lb underweight, I actually did that in 2 stages first I lost just over a stone without trying and was really happy to do that. Then about 18 months later I lost the rest in a very short time. I did have all the other classic symptoms (managed to put my head in the sand and ignore them
)
At diagnosis , my fasting glucose was in the 20s. I had high ketones . The previous day I had had breathing problems when I tried to ride a bike. That's why I went to the doctors.
Nevertheless this could still have been neglected T2.
In hospital (not in the UK) They did tests and scans to eliminate other possibilities such as pancreatic cancer, fortunately all negative. They found very quickly that I was sensitive rather than resistant to insulin . I didn't need very large doses to bring my levels down. They did tests for the various antibodies and insulin production.
Then they really shocked me by telling me I had T1!
Personally ,when I read that someone on the forum has had recent rapid weight loss and very high glucose levels I always worry about a wrong diagnosis .