Mouse over his name and you'll see a lot of information, some of it being how many espionage points he has invested towards you versus how many you've invested towards him. It will show something like e.g. 27/34. The last number is how many points you have against him and if that is larger than the first number there will be a spy icon next to his name.
In other words, it means you have the upper hand against him with respect to epsionage.