I'd start off by putting a wrench on all your suspension, steering, etc bolts to make sure nothing is loose. Grab things and give them a shake to make sure nothing seems loose. Check for any worn out bushings, etc. If nothing seems out of sorts, I'd then get a buddy to move the steering wheel back and forth and even drive forward and backward and look at your U-Joints, etc. You can also jack up the front tire and give them a shake to see if there's any play in your *damn the word is on the tip of my tongue! That should atleast narrow things down quite a bit if you don't find it.