The transition from grid-following (GFL) to grid-forming (GFM) converter technologies is transforming the technical requirements for integrating inverter-based resources (IBRs) into modern power systems. Unlike conventional GFL converters, GFM technologies provide voltage-source behaviour, frequency support, weak-grid operation and black-start capability, making their dynamic performance a key concern for transmission system operators (TSOs). However, manufacturers implement these functions using diverse control philosophies, resulting in heterogeneous dynamic behaviours that require objective and consistent assessment. This paper reviews the principal behaviours currently evaluated during GFM verification, including voltage-source operation, current limitation during faults, temporary GFM-to-GFL transitions, weak-grid performance and black-start capability. It also analyses the evolution of operator verification frameworks towards technology-neutral, performance-based approaches that assess the observable electrical behaviour of converters under representative operating conditions. The review identifies an emerging international convergence towards technology-neutral, performance-based verification principles, with electromagnetic transient (EMT) simulation becoming increasingly important for assessing GFM behaviour. The findings highlight the need to evaluate converters based on measurable electrical performance, supported by validated EMT models and system-level test scenarios, rather than on control-implementation labels alone. These developments are expected to support the secure and reliable large-scale integration of renewable generation and battery energy storage systems (BESS) into low-inertia power systems.