One of the advantages of PBL is the supervisory role of the teacher. Students will have weekly, or monthly meetings with the teacher to discuss the state of the project. The teacher does not only supervise the results produced but the group. They can also assess the group dynamics.
There are clear signs that students struggle with working together. They will complain about uncooperative group members. Another telltale sign is skipping supervision meetings without a good excuse or any evidence of work done. In such a scenario, the group members have no idea of where their peers are about.
There are also other circumstances, where collaboration challenges may not be evident. In such cases, students present a united front and provide the answers that the teacher wants to hear. They will hide the problem until it is time to hand in the projects.
For a teacher it is important to offer comprehensive support. Do not just ask about the group dynamics, but find out whether students are able to plan and divide tasks. If they do not adhere to the plan, why is that? What are the obstacles that they are facing?
Hence, being proactive is the key to detecting issues with collaboration.