While it may be true that there is no way to know for certain what kind of angel your guardian angel is, I think it is possible to make a reasonable guess.
In "The Guardian Angels: Our Heavenly Companions," (from tanbooks), I read how a guadian angel has the power to oversee as many as 200 people, and watch over all of them simultaneously, and yet God gave to each and every person a single angel. Not only that, but each angel only watches over that one person, he doesn't switch to another person once that original guy dies. So this means that each angel has only one person to watch over and that is it, there are no others for him to watch over.
I think we can draw a few conclusions from this. One may be tempted to think that an angel was assigned randomly to each person, but I don't think this was the case. When it comes to the welfare of a man's soul, God never does things by chance or randomness; he's going to pick the guardian angel that is going to be of the most benefit for that particular soul. Each angel is very different from another (angels don't have the same form like humans do, for instance, Lucifer had the form of a dragon), so each is going to have its own qualities that may be helpful to one soul but not as helpful for another.
(This is where it starts to become a matter of opinion). However, I think that the relationship between the guardian angel and the human is one that is very similar to marriage, in which a permanent bond is placed between the angel and the man. The angel wasn't picked at random (much like a marriage isn't random), so I think that the angel's nature and the human's nature are going to complement each other very well. So basically, my primary point really is this: That it is possible to learn more about your guardian angel's nature by studying your own nature. If love and friendship come about by the similarities between persons (which I believe St.Thomas says is what friendship is), then an angel is going to love you more if you have similar qualities to him.
I am of the personal opinion that it is possible to know your angel's nature if you look at the personality you would have had if you had not committed any sins (The personality that God had originally given you). If you have an energetic outlook on life and like to face things head on, your guardian angel might have some form of that quality in an angelic sense. If you are more laid back and like to approach things from a strategic sense, your guardian angel might also have these qualities.
I think this also fits in to the job description of the guardian angel, part of which is to protect you from evil thoughts (which usually comes from the devil) by giving you good thoughts. So, if you naturally get good thoughts that give you a sudden urge to knock down a door with all your strength and adrenaline in order to save a trapped woman, your guardian angel might have that particular personality. If, however, you were given the urge to look around the room for a key instead, your guardian angel might be a more methodical one.
That's what I think. Heh, funny enough, maybe that's why we don't take notice of our guardian angels, because they give us thoughts that we think came solely from ourselves, because it fits into our personality. They blend in so well with us, it's like the guardian angel is wearing camouflage, lol.