The cylinder head casting numbers are located on the outside
of the head, along the valve cover mounting rail and between the 3rd and 4th exhaust ports. The are visible simply by raising the hood and looking at the engine. You should be able to find a cast-in alphanumeric number, possibly one of the following:
Or, it may appear similar to one of the above numbers but a little different.
The cylinder heads used were the D3VE-A2A heads, but after about 1984 the markings of the cylinder heads were not in the normal location at the top of the head. Instead, they were marked on the underside
of the head, beneath the intake ports, cast into the underside of the head, look at the picture below/casting number is on the intake flange (underside) between the pushrod holes in the head:
This is the typical casting number location on 1984-1985 D3VE-A2A's and 1986 E6TE-DA's.