If you find yourself getting winded and no punches are thrown? Moving around constantly? Bouncing? Throwing a ton of "Now, no wait!" punches that go out 3/4 of the way and stop? You are tense. If you can sit back in the pocket and "let it happen" and get a feeling of "seeing everything"? You are sharp.
one of my trainers keeps saying im too tense. every session he keeps saying relax, relax, relax, it sounds like he's trying to hypnotise me half the time
haha same here. like a mantra. ALL the time, I get more of that than all the rest of his advice combined. But it can get kind of confusing when you're not sure whats the difference between sharp and tense, relaxed and sloppy... I guess its just a question of experience.
It's pretty simple, the idea of punching is to contract your muscles in the correct order. When you can do that well you're sharp, you have timing and coordination. Your power and speed will be good. When you're tense you're contracting muscles when you shouldn't be, you're wasting energy, you'll be slower than you should be. You're less coordinated. It's just a matter of learning to punch correctly, practice and you'll feel yourself improve and become less tense.