This is a feature which I have been seeing on and off for a few months so I’m not sure if it is one of their experiments or if it is being rolled out gradually. It’s very simple: advertisements that appear at the top of your results lists and in the panel to the right are marked with a little yellow box with ‘Ad’ written inside.

Over the years it has become harder to identify ads at the top of results as the pale pastel backgrounds to them became more subtle. It has been suggested that the more obvious marker is a consequence of discussions between Google and various regulatory authorities.