Look on the emissions sticker under the hood. The displacement should be listed near the bottom of the sticker
If that is missing or has been painted over, look at the dist. Does it come out of the head near the front of the engine, or stick out the side of the block part way back?
Dist in the head = Mazda 2.0L (wide cast aluminum valve cover)
Dist in side of block = Ford 2.3L (narrow stamped steel valve cover)