In this paper, a comprehensive assertion-based verification methodology for the digital, analog and software domain of heterogeneous systems is presented. The proposed methodology combines a novel mixed-signal assertion language and the corresponding automatic verification algorithm. The algorithm translates the heterogeneous temporal properties into observer automata for a semi-formal verification. This enables automatic verification of complex heterogeneous properties that cannot be verified by existing approaches. The experimental results show the integration of mixed-signal assertions into a simulation environment and demonstrate the broad applicability and the high value of the evolved solution.