Monoepoxidation of 1,2-dimethylcyclohexa-1,4-diene using m-CPBA
The interaction of the alkene with m-CPBA is primarily nucleophilic/electrophilic in nature, i.e. the π MO of the alkene attacks the O–O σ* orbital. Therefore, the more electron-rich alkene (i.e. more highly substituted alkene) preferentially reacts.

With methyl groups there probably isn't too much steric hindrance. m-CPBA approaches above/below the alkene and doesn't directly attack over the methyl groups.

Indeed, the monoepoxidation of 1,2-dimethylcyclohexa-1,4-diene is reported to proceed selectively at the more electron-rich tetrasubstituted double bond (J. Org. Chem. 2006, 71 (4), 1653–1657):