Finding Ads on a web page

I'm writing an application that's trying to determine if there are ads on a page. This is currently using brower-driving through selenium webdriver using python.

I figured that a good amount of ads exist inside iframes, and I've made a loop to look inside each frame

browser = webdriver.Chrome()
browser.get("http://cnn.com")

all_iframes = browser.find_elements_by_tag_name("iframe")

for iframe in all_iframes:
   browser.switch_to_frame(iframe)
   print(browser.page_source)
   browser.switch_to_default_content()

browser.quit()

I'm wondering if there is any consistently found tags or tag parameters that I can use across multiple pages to determine if there are ads located on a page (both in and outside of iframes on a page). Would I have to look for instances of stuff like doubleclick or adtech or adblade inside each frame?

Or would I have to generate different rules for checking on a per-page basis?

Anyone in the know about how ads are displayed on pages? Thanks.