Just to help visualize -- a rough sketch of what's been looked for in terms of "lines" -- the "point" where lines meet are actually joints and so should always occur solidly on the dog. I can be easy to draw the lines where you think the bones are then end up with points that are float outside the dog. So what many do is find the joints (the yellow dots) and play connect the dots.
You want to be able to see at a glance the angle of the shoulder, the upper arm, the pelvis, the femus, and the lower leg, as well as where they connection.
There is of course a lot more to structural evaluation in case that's not clear...we haven't touched on rib cage coverage or forechests or width or any of the various topics...the pitbull link provided earlier in the thread does talk about a lot of that. There is also an ebook written by the author of adojrts' link, Helen Grinnell King, called Picking Your Performance Puppy. It is written from the perspective of agility but there are accompanying videos and such to help you see the results of different structures and give you a starting place in evaluating your own and other dogs.
For any dog-parkers and dog-sporters, you have a perfect opportunity to watch dogs of different shapes and sizes use their bodies in different ways -- see whether they choose to trot or pace, how clean their movement is, acceleration vs. speed vs. turning ability, how easily they lower their center of gravity or perform odd tricks like standing on their hind legs for extended periods of times. There is no ideal structure -- just ideal structure for a given job -- because there are
always trade-offs.