when making a swimbait, how do you know that when you put all the pieces together that its going to swim the way you want it to?

trial and error

Yep trail and error and make sure there is enough room between the joints

ToledoEF is right but some general guidelines exist.  There's a current thread on tackleunderground/hardbaits about lip angle that discusses some of them.  The best advice I have is take a look at some of the popular swimbaits being sold commercially - shape, weight, how the segments are proportioned, the lip.  Use that as a starting point because the commercial baits have been extensively tested and tweaked before bulk manufacturing (hopefully!).

While there is some trial and error involved, if you will do some homework and study other sucessfull swimbaits it will save a lot of the error portion of your work.

This is a personal opinion and I realize it is not shared by everyone on what a true swimbait is but if it requires a lip for action, to me, it is a large crankbait.

To me a true swimbait has action without the aid of a lip. For example if you were to look at an MS Slammer, its called a swimbait, but if you were to look at a Mouldy's Hawg Wobbler it is refered to as a musky crank/topwater bait and they are nearly identical in design. The Mouldy has been around long before the swimbait thing was even known.