You can really dig deep and use some special tools to see what the best audience is for you to optimize your ads.

For instance, Google has an Ad Planner Tool, which they also call a "media planning tool".  It helps you understand your reach,

relevance, and insights (learn more in video below).

The Google Ad Planner Tool Helps You:

Identify websites your target customers are likely to visit

Define audiences by demographics and interests.

Search for websites relevant to your target audience.

Access unique users, page views, and other data for millions of websites from over 40 countries.

Easily build media plans for yourself or your clients

Create lists of websites where you'd like to advertise.

Generate aggregated website statistics for your media plan.

This video gives a good strategic overview of reach, relevance, and insights.