You can search for different formulas for fit that will take into account all sorts of measurements.
If you want an idea of how you can measure yourself and use that to help determine a what range of measurements you should see in a frame that will fit you, try reading this fit article from Peter White. It is long and maybe dry to most but it is a good primer into the philosophies of bike fit.
This article will get you more practical results quicker and might be more of what you are looking for. In the end you might find that the basic Trek fit guide will match what the more complex formulas tell you. It is good to find the potential problem areas before you buy though.
One other option is to go to a shop that will do a fit for you. It will not matter that they do not carry Trek, your measurements will point them to bikes of similar sizes that you can use as a guide to what size Trek will fit. Trek has the geometry for each bike available on their website.