Hello, thank you for your question. You are correct that there is no definitive test for this issue. They do EMG and nerve conduction studies to help in the process of diagnosis. Skin biopsies are used to confirm loss of cutaneous nerve innervation as part of the process. Diagnosis therefore tends to be based on characteristic symptoms and is most often confirmed with a skin biopsy demonstrating reduced intraepidermal nerve fiber (IENF) density. Through this means, they find nerve density reduced in patients with small fiber neuropathy and in particular in the Intraepidermal nerve fiber. In SFN, this can be reduced 65 to 90%. They also take into account where the neuropathy is and automatic nervous system involvement. Underlying disease is considered such as diabetes. Fibromyalgia is also often commonly related to SFN.
To date, they do not know the exact cause of ulcerative colitis. I am wondering if your doctor is not from the era where ulcerative colitis was always blamed on stress, diet, etc. Today we know that it also can be autoimmune related. UC happens when the autoimmune system attacks healthy tissue in the gut which then causes inflammation in the gut leading to symptoms of UC.
Have you been diagnosed with Dermatomyositis and Gottron papules? This can involve not just the rash but progressive muscle weakness and on both sides of your body starting at the trunk moving out. This may be an area to talk to a doctor about as peripheral nerve involvement in dermatomyositis has been known as neuromyositis. Not all doctors will correlate these two things but there is literature to support it. Here is a study that you can copy and paste into your browser to to read about this. https://pubmed.ncbi.nlm.nih.gov/20229079/
Hopefully you will have a new doctor or specialist that you can receive a proper diagnosis and treatment with.