One of my users let the cat out of the bag and told me they were using one of my free apps, which is monetized by ads, but they were blocking the ads with an ad blocker. They told me this mockingly, as if I can't do anything about it.
Can I do something about it? Is there a way to detect that ads are being blocked?
There is nothing you can do that your users can't do better.
The only thing that comes to mind as remotely effective is to make the ads an inextricable part of the program, so that if they're blocked the user cannot make sense of/interact with the application.
Can you check to see if the ad loaded in your app?
Ad blockers work by preventing your app from downloading data. You could check the content length of the data in your ad frame to make sure there is data there.
If there is no data throw up a message and exit or warn you with an email.
It might not be as big an issue as you think since only a small percentage of people block ads.
I think it depends on the content provider for the ads. I know the AdMob SDK provides a callback when an ad request fails. I suspect that you might be able to register for this, then check for a connection in the callback - if there is a connection and you did not receive an ad - take note, if it happens more than once or twice, chances are likely your ads are being blocked. I have not worked with the AdSense for Mobile toolset from Google but it wouldn't surprise me if there was a similar callback mechanism.
This is an extension of a previous answer. The user has informed me that the app they are using is called AdFree Android. It can be found on the market. The app says it works by "nullifying requests to known hostnames serving ads."
I suggest that if you monetize any of your apps with ads, you check at startup for this program and give the user a nasty message, then terminate your app.