I was sent to the Lymphedema Clinic through my health insurance because I had complaints of my arm going completely numb at night (couldn't feel it, dead weight). The nurse at the clinic took measurements of various parts of my body. Confirmed the diagnosis, taught me how to do lymphatic massage, wrap my arm, and then sent me to get fitted for a sleeve. That was about it.
My surgeon took baseline measurements of my arms before my mastectomy/lymphovenous bypass surgery and they measure my arm at every office visit. There are several visual "cues" to diagnosing LE, along with patient complaints of swelling, pain, "heaviness" in the arm or other area. Procedures such as lymphoscintigraphy are typically used before surgical treatments as far as I know.
In addition to the helpful responses above, these pages from the main Breastcancer.org site might be helpful:
I literally just had a pre-op Occupational Therapy appointment this morning. She went over exercises and took measurements of both of my arms to establish my baseline. My lumpectomy is Aug 29th with Sentinel Lobe removal. She wants me back 5 weeks after surgery to make sure I’m doing OK. Gave me specific instructions to get in touch with her sooner if there’s any signs of Lymphedema. Exercises are to start 2 days after surgery. She was very thorough.