Put these together and your problem goes away. Your question isn't whether a project team member is on track. The question is, for the tasks to which the project team member's name is attached, are they done or not done? The possible answers are yes and no. That's it.
[ If you like this project management advice, there's lots more where it came from in Bob's classic on the subject, "Bare Bones Project Management: What you can't not do." Order your copy today. ]
Unless a project team member simply lies to you, you have a definitive answer as to whether the project is on track: If every task on the critical path that's scheduled to be completed actually has been completed, the project is in the green. Otherwise, it's in the yellow.
If you find that a team member does lie to you, telling you a task is finished that in fact is still in progress, it's probably time for a personnel action -- a documented verbal or written warning. Tasks slip. It happens. Deliberate deception doesn't just happen and you can't allow that to be OK, for a very simple reason: It isn't OK.
This story, "Tasks and timelines: The keys to successful project management," was originally published at InfoWorld.com. Read more of Bob Lewis's Advice Line blog on InfoWorld.com.