Diagnosis of ERMS typically involves a combination of imaging studies and biopsy. Imaging techniques such as MRI, CT scans, and PET scans help to determine the size, location, and extent of the tumor. A biopsy is essential to confirm the diagnosis by examining the tumor cells under a microscope. Immunohistochemical staining and genetic testing may also be used to identify specific markers and genetic abnormalities associated with ERMS.