It seems to me that the days have gone when you could have resources fully allocated to a single project. The intent is to share resources and squeeze as much productive time out as possible across a large numbers of projects.
So the question is when are resources allocated to too many projects ? The answer is when they feel that just standing still juggling with all the different tasks means they cant make any forward progress.
As a Project Manager how much attention do you pay to your resources ability to focus on the tasks at hand and get them finished? How many times do you see a task in the project plan sit at 98% for weeks on end?
As a Project Manager your team is the key to success and so we need to spend time talking to them about their ability to focus on the project related tasks and how, as the Project Manager, we can make it easier for them to do that?
Another way to help is to have visibility into your planning tool to the allocation of time to a resource and be constantly updating it to reflect a realistic view of the tasks at hand and effort required.
So who makes it a priority to spend one on one time talking to project resources and asking about how they are dealing with the project work and what you can do to help ?