How to test a complex integration with multiple external services with minimal documentation?

When testing complex integration with multiple external services and minimal documentation, the following approaches should be used:

• Studying service behavior through observation: analyze real requests and responses, logs to understand the format and logic of interaction.
• Creating mocks and stubs: emulate external services to isolate the system under test.
• Step-by-step testing: verify integration with each service separately, then together.
• Using contract testing: record and verify interaction contracts.
• Feedback from the team: clarify details with developers or support of the services.

This approach minimizes risks and increases the reliability of the integration.