You may think I'm a douche. I really don't care. Here is my problem with knowing your opponent's skill level, especially in leagues: It is of little help to you in assessing how good your opponent is, and is usually flawed, due to people like you and your captain. Before you take exception to that, here's the explanation.
Let's say you're a 5. Your opponent is a 4. Now, it is rare to have a player at a level lower than a 6 that will concentrate on every single shot, and not play the score. If you're playing a lower ranked opponent, you subconsciously have already chalked up the win, because, according to the rankings, you're better than them. You may watch them, and figure out their strengths and weaknesses, but all you will discover is (a) they don't play as well as they should, (b) they're fairly ranked, or (c) they play better than their ranking would indicate.
If you're playing a higher ranked player, you will focus on playing better, as you know you won't have as many opportunities at the table, and will get punished for your mistakes. You'll also be playing with some nerves, as you have already mentally chalked up the loss, and start playing the score instead of the game. "Well, I'm down 2-0, but he needs to get to 5, so it's a 3-3 race". Or, in the case of 9-ball, "well, he's up 30-3, but I only need 28 balls to his 65". That isn't the correct way to play pool. It's the correct way to play "handicap management" pool, but not pool. You end up playing loose against lower ranks, and tight against higher ranks. So your handicap isn't a true reflection of how you play. It's a reflection of your chosen strategy against the majority of your opponents.
Give every shot its due attention. Play every opponent as if they can run the table on their next attempt. If you are a true competitor, you owe it to your opponent to play your best game. And you'd expect the same from them. Suppose your opponent shot one handed with their opposite hand against you in league, because they were pissed off at their asshat of a captain. It's like showing up to work and doing a poor job because you think your boss is a jerk. It's unprofessional. And it ends up being a reflection of how you will conduct yourself in life.
The only way you will get better is to go out and play everyone as if they can destroy you on their next shot. This way, you don't start playing loose because you don't think your opponent can beat you. That attitude is disrespectful. And I loved it when one of my players (yes, I was a captain...but I always told my players who they were playing, what their race was, and even updated them during the match if they wanted a count, or score) would come back after a loss and complain about the sandbagger that just beat them. Sometimes, it happened. Most of the time, they just took the win for granted, and gave the lower ranked opponent no respect.
Play the table. Respect your opponent.