This is actually my question. How to know that your websites will be in a good position before you put it up? I don't want to waste my time on site that will not rank in search engines.
First of all, there is no definite way to tell before hand. but look for these things. do you get a lot of traffic ? does your website have a lot of quality content ? if you can answer yes to those then by all means you can profit well, by the way, it doesn't hurt to put up adsense anyways and see how it does.
It depends on what you think is "profitable" Find niche. research highest adwords paying keywords, estimated global traffic, competition, that's it.