The CX has an 8 valve head and cat converter in front of the engine, the VX a 16v VTEC-E head and cat infront of the engine and the DX a normal 16v head with an under-car cat. If you can't tell the difference by that:

Look at the engine block stamp, halfway down beside the exhaust manifold.
D15B8: CX
D15B7: DX/LX
D15Z1: VX
(D16Z6: Si/EX)