Okay, so you do kw research and grab some kw with high CPC. Then when you get a code from Adsense to place an ad block on your site, how do you know what kw that ad is going to represent?
It determines what ads to place by the keywords used on the site. If they don't have ads relevant to your keywords than they will only place generic stuff.
You won't know. Neither will any human being, with 100% accuracy. Google's algorithm matches the ads in their inventory with your site's general theme/niche, keywords on the page, preferences of the visitor, etc.
Is there a site where you can identify what keywords people are searching on so that you can identify a high ranking keyword and then use that for Google Adsense?
If you see a $4 CPC keyword, as long as you are not smart-priced, you can expect, say, $2-3/click. Some visitors may be shown ads based on their preferences, but overall, you can set revenue expectations based on the CPC shown. It may fluctuate wildly depending on your niche and your user demographics.
My suggestion: Continue with your journal. Get some real eCPM data for your niche from Adsense interface. Then you won't need to ask these questions.
Is there a site where you can identify what keywords people are searching on so that you can identify a high ranking keyword and then use that for Google Adsense?
you build a page for a specific keyword
adsense has a bot that reads your page and understands what's it about, so relevant ads will be displayed when you place your code on that page
the only way you have to control which ads are displayed is editing your content