Find your local gokart club (not Gasoline Alley - find the guys that RACE them). Go to their meets and start asking questions. 9 guys out of 10 will let you drive their karts so you can see what different ones do. Inspect 'em, take pictures, etc.
You should be warned, however, that whatever you make won't be competitive against the purchased gokart frames. You simply don't have the financial backing the factories do, you don't have the tools, and you don't have the huge, highly compensated engineering staff
