There are several methods of usability testing for GUIs (Graphical User Interfaces):

1. User Interviews: One-on-one interviews to gather feedback and understand user behavior.

2. Heuristic Evaluation: Expert review of the GUI against usability principles and guidelines.

3. Cognitive Walkthroughs: Step-by-step analysis of user tasks to identify potential issues.

4. Usability Testing (Moderated): Facilitated testing with real users, observing and gathering feedback.

5. Remote Usability Testing: Online testing with users, often using screen sharing and video conferencing.

6. Unmoderated Usability Testing: Automated testing with users, often using online tools and surveys.

7. A/B Testing: Comparing two versions of a GUI to determine which performs better.

8. Paper Prototyping: Testing low-fidelity prototypes to gather early feedback.

9. Guerrilla Testing: Informal, in-person testing in public spaces or offices.

10. Contextual Inquiry: Observing users in their natural environment to understand their needs.

11. Surveys and Questionnaires: Online or offline feedback collection through structured questions.

12. Accessibility Testing: Evaluating GUIs for usability by people with disabilities.

These methods can be combined and tailored to suit specific project needs, providing valuable insights to improve GUI usability.