Top lane main.
This is especially true the very first levels. If you see your lane opponent use a skill to farm, go aggressive. 2 skills vs 1 skill almost always win.
Example: I play Jax vs Renekton. We both hit lvl 2 and I see him use Cull the meek (his AOE ability) to farm minions. I jump on him and use my 2 abilities. If he decides to trade back he will loose because he only has one of his abilities to answer with. this is an example how a Jax, a champion that is much weaker and should loose early game to Renekton can turn it around and get an advantage, maybe even a kill if Renekton stays to trade. One thing you need to think of though is that if he has a big minion advantage you should not go for the trade. Early, the minions will do tons of damage and make up for the damage Renekton loses on not having all his skills available.
This stays true in many cases. Not every of course. Like if you play Malphite vs Renekton. Renekton will probably still win the trade even if he just used a skill to farm. Some examples on champions that often use skills on minions and that you should look to trade with when they do: Renekton, Nasus, Riven, Shen, Shyvana, Lee Sin. If you don't know the approximate cool downs of your enemies skills you should check it up in the loading screen.