Easy.

Top, to bottom:

The heads are completely different. The B18A uses an updated DOHC ZC style PR4 head, while the B18C uses a newer P72 DOHC VTEC head.

The B18C has the larger head locating dowel pin holes on the opposite side of the B18A to prevent accidental DOHC VTEC head swaps.

The B18C has an in-block VTEC oil supply passage, in-block oil squirters, and a modified lower end with a stock crank girdle.

The crank, rod, and piston sets are different also, however are interchangable as a matching crank/rod set (B18C crank and rods in a B18A or B18A crank and rods in a B18C.)