We propose to employ element-specific x-ray magnetic dichroism to investigate the local spin configuration in thin films of RuO2, a prime member of a new class of compensated magnets: altermagnets. While the existence of linear dichroism in compensated magnets is well established, we aim to experimentally demonstrate that, apart from the ferromagnetic or non-collinear order, circular dichroism can also originate from the collinear altermagnetic order resulting in a spin split band structure. The proposed experiment will exploit the unique capabilities of beamline ID12, offering large magnetic fields required to manipulate compensated magnetic materials, and full X-ray polarization control.