The isomeric relationship between cis-1,2-dichloroethene and trans-1,2-dichloroethene is that they are geometric isomers, specifically, they are diastereomers. Geometric isomers are molecules with the same molecular formula and connectivity of atoms but differ in the spatial arrangement of their atoms or groups.In the case of cis-1,2-dichloroethene and trans-1,2-dichloroethene, both molecules have the molecular formula C2H2Cl2 and the same connectivity of atoms. However, they differ in the arrangement of the chlorine atoms around the carbon-carbon double bond.In cis-1,2-dichloroethene, the two chlorine atoms are on the same side of the carbon-carbon double bond, while in trans-1,2-dichloroethene, the two chlorine atoms are on opposite sides of the carbon-carbon double bond.To determine the isomeric relationship, you can analyze the structure of the molecules and compare their spatial arrangement. You can also use spectroscopic techniques such as infrared IR spectroscopy, nuclear magnetic resonance NMR spectroscopy, or mass spectrometry MS to differentiate between the two isomers, as they will have distinct spectra due to their different spatial arrangements.