How do you know when enough is enough?

I recently wrote an entry on LinkedIn APM PMOSIG group entitled ‘How does one go about determining what skills and how many staff you require when setting up a PMO from scratch? Surely answer isn't just simply 'refer to P3O guide' ‘. This stimulated some good discussions as well as some links to useful information.
 
Whilst reading and digesting the entries to the blog, I began wondering how does one 'know' that one has the appropriate skills within a department, or how does a project manager 'know' how much assurance and how much scrutiny has to be applied to his/her project?
 
Project managers sometimes have to struggle with providing this assurance effectively and efficiently whilst trying to avoid imposing unnecessary burden on themselves and the team? 
 
Stakeholders require assurance from project managers that their programmes and projects are being managed effectively and efficiently and will deliver on their objectives.
 
The need to ensure that projects succeed is making sponsors look at the nature, extent and cost of assurance.
 
So how does the project manager know when the assurance being provided to them is adequate, accurate and appropriate? How does the project manager assure themselves that accurate information is being provided from within the team?
 
The project manager is pivotal in monitoring and control of a project, but has to rely on team/work-stream leads (especially on larger projects, or where teams are not co-located) to provide the data so that assurance can be provided to stakeholders.
 
Is the solution: good leadership? Good communication? Good plan? The project manager being a control-freak? All of the above?