“First, how do you determine their age and background? Do you ask them directly?”
If you have a big email list, then you probably have a good amount of traffic coming to your site. In that case, you can use a site like Alexa.com or Quantcast.com to figure out the demographics of your visitors. Look at this blog post for instructions on how to use those two websites: http://www.keywordsblogger.com/find-your-audience-how-to-identify-a-niche-audience-to-serve/ . Otherwise, you can send a survey out to them via email, or you can simply infer their age/background by reading the emails they send to you. That’s how I’ve determined the demographic that buys my products. They’ll often say in their emails things like “I’m 52 years old and overweight…”
“How do you then determine if they would be interested in your next promotion, say, hair loss treatment? Do you just guess and send them the informational/promotional emails anyway?”
You can ask them explicitly what types of problems they’re having in a survey, or you can email them offers, and see what % of them (1) open your email, and (2) make a purchase.
“And second, how do you determine how much Adsense action you put on a site?”
All of your Adsense questions are addressed in the resources I’ve already pointed out:
– The Ultimate Heatmap: http://www.dynamicmedia.com/ultimate-heatmap/
The best way to answer “what will happen if…?” questions is to test. Time to start implementing, Alwyn!