Project Management is actually a discipline, and there are many that get the appropriate certifications.  

Making 'proper' decisions translates to making the best decision based on the information available at the time, which may in fact need to be altered as conditions change.  Fundamentally, a project manager is weighing three sides of a triangle - scope of deliverables (functionality) vs resources available vs delivery schedule.

Trade-offs start when one of the three sides is fixed in stone by someone with higher authority.  In other words, if the schedule is fixed - "We *will* release the next version on March 31st", then the PM needs to be able to alter the other two sides - can I get more resources (generally more developers or testers)?  Can I limit the deliverables? Tools used include PERT and Gantt charts.  PERT is used to show dependencies and critical paths, Gantt tend to be drill down as you identify each/every subtask within a task to track at a more refined level.

I agree with Jerrold Heyman. The success of decision-making depends on the objective and can change over time. As such, success of decision-making is context dependent.

I prefer to use the word effective instead of efficient, because effectiveness relates to the effect a decision-maker tries to accomplish. There is no such thing as a 'best' decision; it depends on how the objective is stated and perceived. Cost-effectiveness could be an evaluation criterion to check the success of decision-making. Cost-effectiveness is the ration between input and effect.

I would like to add something regarding the human nature of projects and of project management.

There are a lot of human factors to be taken into account for decision making for the management of a project. A decision that could be seen as sub-optimal from purely fixed criteria could turn out to have been the best decision later on, because the manager knows he is dealing with humans, who can fail, who can get ill, who needs to be satisfied with their tasks, etc.

Another addition I would like to make is that, when you are looking for an 'efficient' decision, you need to decide along which criteria you want to optimize your decisions. Indeed there are several ways to optimize a project, since in real life a project has much looser boundaries than in methodologies. You can optimize your project for delivery time, or for product reliability, or for the team skill acquisitions, or for product ease of extend-ability, ...

Hence project management is an intricate process of decision making that becomes even harder to model when you start adding new dimensions such as human factors, etc.

Hope this helps.

Thanks to Jerrold, Wouter and David for your responses. 

I would like to add a small point to the discussion. Since the question directly relates to the identification of the ability of a project manager to make decisions, I would think one of the crucial characteristics one could look out for is the extent of objectivity that the person exhibits when dealing with problems or crises. For instance, if the person gets flustered because of problems such as customer dissatisfaction at some point, then perhaps to that extent he / she will be handicapped in making effective decisions. Can the person maintain poise and calmness and objectivity at times when problems arise - I think this is a critical indicator that very often sets apart the effective project manager from the less effective ones.

Thanks Meenakshi for your point.

What I have done in the past is to identify the "known" expert practitioners in the field - - -  thus it is by consensus that we do this.  Might be many other ways; however, this one works best for me.

would be interesting to perform a study on project managers where the objective is to see the correlation between attributes (maybe even cognitive attributes) of the managers and the projects success. Is this what you are planning to do?

One of the outcomes can even be something like a project management aptitude or cognitive test.

@Nordin Zakaria and @Jerald Feinstein thanks for your points.

@Nordin Zakaria your statement is quite interesting as to what we want fro this discussion, I would appreciate if you explain even more. In fact, the relationship doesn't necessarily need to be a correlation one, perhaps it could be casual relationship. 

Project manager must have the ability to solve problem accrue in the real time during project processing, that is why become a manager after years of experience to be a senior or project manager . I think this is the most important point .

You need for efficient decision makings in software projects just the best Web Developers/-Programmers. Or even better Lien Developers, and of course detailed user requirements for the whole development process. The rest is like butter... ;) o, and i forgot you need for that just about 1Mio.

PMI has said that Soft Skills are the core of best PM, imho, being an excellent communicator is the most important attribute of all.  

I've been a Software Project Manager(PM) for quite a few software projects in my career. One objective measure that the PM is good is obtained by tracking the amount of rework/ slippage involved in his/ her projects. The lower the rework and/or slippage in effort, the higher he/she should be rated because if things were perfect/ideal, we should clearly get rework/slippage effort=0! This a kind of 'top-level' or key indicator. Low rework/ slippage values can result only if the PM is experienced, a good manager, one who can handle difficult team-members, has done a thorough job of requirements analysis, has planned correctly, monitors progress effectively and so on. To track rework/ slippage, you need to aggregate time-sheets submitted by team members and compare with planned/ budgeted effort on an ongoing basis.