Is there one best test to diagnose ewing's sarcoma?

Not one. Ewing sarcoma is relatively rare tumor mostly seen in young adults 10 to 20 yrs age involving the long & flat bones, presents with pain swelling , 30% of the time already spreads. Simple x ray will show bone lining ( perosteal ) reaction, MRI , ct , bone scan , pet scan , are all diagnostic tools , confirmation is by biopsy .
Biopsy of lesion. Biopsy of the lesion and examination of the tissue by a pathologist is the best and only way to diagnose ewing's sarcoma.