Use a DICOM viewer
Open the DCM file in a medical imaging application
Use RadiAnt DICOM Viewer
Use MicroDicom
Use Weasis
Use Horos on macOS
Use OsiriX on macOS
Use 3D Slicer
Use ImageJ with a DICOM plugin
Use a PACS system
Convert the DCM file to JPG or PNG if needed
Check that the file is not corrupted
Make sure the viewer supports DICOM format