There are many tell-tale signs that your weighting is off underwater. For instance, you may balloon to the surface near the end of the dive. There are other, more subtle clues that your weights could use some fine tuning. One of these is how streamlined you are in the water.
Streamlining refers to your ability to maintain a horizontal position in the water. When you're overweighted, you have to compensate by inflating your BCD. This alters your center of gravity to the point that you are swimming with your chest high in the water (and your legs low).
When you're underweighted, you have to continuously kick down to keep yourself at depth, resulting in a profile position with your legs high in the water.
See the figure below for how each of these looks in the water. Also,
take a moment to make the mental connection of how you might feel in
each of these positions. This way, if it happens you become aware of it.
Like I said, it can be subtle.
[Click to enlarge]
Correcting this problem has many great effects. First of all, you can more easily enjoy your dive when you are perfectly streamlined. Secondly, it'll improve your air consumption. It'll also reduce the likelihood that you inadvertently kick any aquatic life.
Fixing the problem may not be so easy as putting more weights on your belt (or taking them off), although you will want to try this at first. Play around with the location of your weights and see how it affects your balance underwater. You may find putting weights in trim pockets is more helpful.
For example, I use a weight-integrated BCD with around 10 lbs of weight (in tropical water). Most people might put 5 lbs in each weight pocket. However, I prefer 3 lbs in each trim pocket on my back, and 2 lbs in each weight pocket. This took experimentation and about four dives for me to fine tune, but it was well worth the minimal effort.
Also, if you find your legs are especially buoyant, you may want to consider using ankle weights to bring them horizontal. Basically, it comes down to observing your body underwater. Remain still for a moment and feel where your balance is, then think about how you can shift weight to make yourself centered. You will be amply rewarded in future dives.
[Click to enlarge]: http://thedivingblog.com/uploads/2010/04/weighting.png