there is an ask a question get an answer thread for your this type of thing...but for some reason people don't seem to enjoy using it...whatever

anyway, Toph can see her opponent with her ability to detect vibrations. The way earthbenders work is they generally strike the ground somehow to begin an attack. Toph can see the motion they make and can therefore infer what attack they will use against her...if the earthbending opponent tries to alter his attacks movement they still must be on the ground and make some kind of motion
Toph will see that motion and know what they are doing.

basically she doesn't see the attack, but she does see the bender preform it, so she still knows what to do to defend herself

I agree with LordMordor's opinions on this, but I also think you have to give a little in this case. It is a fantasy show, and although that is not an excuse to allow a blind chacter to temporarally see in order to make a situation easier for the animators, writers, or whatever the case may be, I don't think that the nitty-gritty, one time details are as important. Not to belittle it at all of course.

Toph ability to see flying objects my be based on the reactions of your body on the ground your standing on. Such as shifting your weight, and so forth. Then she would interperet where the object is going, and how fast.