Projects have different tasks. As PM, you need to measure how effective the team member with the task he/she is executing. What if the team member multitasks? How do you measure multitasking? I have no idea.
Are you doing EV on your projects? If not, implement it. With it, you can measure your cost and schedule (to a point) performance of your work packages. Analyzing your indices, you can narrow down where you are over or under performing and then dig deeper from there.
If the team member is multitasking, then his or her work falls into one of two categories:
- Work that you have handed out;
- Work that you have not handed out.
When you hand out a task, you should ensure that you agree a delivery date for that work, and monitor whether that delivery was achieved. Initially, at least, you can probably ignore the work that you have not handed out (unless the team member is taking work on without authority, which is a completely different issue.) Over time you will get a good understanding of whether the team member is usually achieving what has been agreed or not. If he or she is, then you have no real problem. However, if not, then you need to investigate whether the other manager (ie of the other team that has a call on the team member's time) is also concerned. At that point, it becomes a discussion between the two managers.
One further concern is that the team member could be playing the two teams off against each other, so even if the work is being done, you should probably still meet with the other manager to establish whether you both think you are getting a fair day's work from the person... If not, you may have to start monitoring performance on a more regular basis to find out what is happening.