How does one determine which stocks will be profitible and which ones will not?

[raysor]

It's called knowledge!
Firstly you have to realise that most of the published data, research, recommendations etc. has a hidden agenda. So you never follow it blindly. Use it as a source of information but do not just follow it. Secondly the market is trying to discount the future so basically current prices have taken into account future (6-12 months?) events.
Thirdly avoid smaller companies as these are very vulnerable to lots of events. For example a small oil explorer: Whilst it is exploring there will be lots of speculative hope and the share price will be way over-valued. The odds of striking a big oil find are quite remote but if it is found that there is less oil than speculated, or un-commercial to extract then the shares will be worth next to nothing.
So I would suggest the leading companies that have strong government, strong balance sheets (lots of cash and assets), good cash flow, a product or service that is always in demand, and a business that is relatively easy to understand.
Fourthly, always buy for a reason, even if your reasoning is unknowlingly flawed and then sell when that reasoning is no longer valid or comes to fruition.