There are quite a few ways to differenciate the different models.

The HF has no clock in the middle of the dash, steel wheels, a small front lip, and an HF badge on the back of the car.

The DX(or just the CRX model)has a little different wheels, clock in the dash, no model sticker on the back but no front lip.

The SI has a sunroof, aluminum wheels, front lip, sport seats(higher bolsters, cutouts in the headrests), clock in the dash, si badge, rear wiper blade on hatch, and "CRX" stiched into the door panels in a fabric piece.

Those aren't the only things that can differenciate the models, but those are the obvious ones. Check out my CRX history page at http://www.poodus.com/crx/crxinfo.html

Hope this helps