The kill counter in the top right corner tells you how many kills your team has, and how many their team has. Generally, if your team has more kills, this means they have more gold, and therefore, better items. In some cases this might not be the case, so really just see how you and your team have been doing the entire match, look at how much cs you/your team has compared to them, and really the most important is items the enemy has. If you are playing as an AD carry, even if you have more kills, if the enemy AD carry has been farming passively all game and has a full build but no kills, you're going to lose unless you also have a full build, and even then, the other AD carry might just be a better late game champion then you are. A fully built Caitlyn will just flat out lose to a fully built Vayne no matter how fed one or the other is, and this is just knowledge you gain from playing the game more.