Once you're around 1.3 you enter top 5% but that doesn't mean much. In reality, so much of the destiny population is absolutely terrible at pvp. There's no true way to measure how good someone is until we have custom/ranked matches. There are guys with KDs under 2.0 who are far better than players with 2.0+. There are also some players that are 2.0+ that are as good as their stats suggest. There's no concrete way of making the decision. If you want to generalise however, if someone has a KD over 1.5 with a decent amount of kills (15k) or so, you can be rest assured they will punish you if you're playing badly. That's about all it really tells you.