Oldsmobile
A quick way to differentiate between small-block (307, 330, 350, 403) and big-block (400, 425, 455) cylinder heads is by the casting identifier. Small-block heads are marked with a number cast into the lower left corner of the head over the exhaust port; big-block heads, like this one on a 455, have a letter in the same location.
The casting number on Olds engines is located at the front of the block on top of the timing gear housing. A nine-digit engine ID code is stamped on a pad directly below the Number One cylinder. The first digit is a 3, signifying the Oldsmobile division, followed by a single digit for the year, e.g. 2=1972, a letter designating the assembly plant, and a six-digit serial number that will match the VIN if the engine is original to the car. If you're searching for an Olds 455, here are the casting numbers to look for: 396021F ('68-'71); 396021Fa ('72-'76); and 231788L (service replacement).
Buick
Buick 350 and 455 engines can be identified by the location of the engine serial number on the block without actually decoding the number. On 350s, the serial number is stamped on the front of the block below the driver-side cylinder head. On 400s and 455s, the serial number is stamped on the block between the first two spark plugs and the exhaust manifold on the driver side.
On a Buick 455, the displacement is cast into the top of the bellhousing flange at the back of the block. On 350s, it's at the back of the intake manifold.
AMC
The AMC 290, 304, 343, 360, 390, and 401 all share the same external specifications, and the intake manifolds interchange. They can be identified by the cubic-inch displacement cast into the block on either side. The number is under the engine mount brackets, so you may need to feel it rather than see it.
Pontiac
On most Pontiac 350, 400, and 455 blocks manufactured after 1968, the cubic inch displacement is cast into the driver side of the block just in front of the first freeze plug (arrow). On 428 blocks, the displacement is cast upside-down near the center of the block, also on the driver side.
A three- or four-digit date code is located on the distributor pad at the back of the engine. The first letter indicates the month (A=January, B=February, etc.), the second and third digit correspond to the day of the month, and the last digit is the year. The last two digits of the year are also cast in large numbers near the distributor hole. Somebody screwed up on this 455 because one date says '75 and the other says '76.
Here are some other tips for dating and identifying Poncho blocks: (A) Pre-'67 blocks have two freeze plugs per side, while later blocks have three. (B) Prior to 1970, blocks were drilled and tapped with two motor mount holes; after '70, there were five holes, but on some blocks, only three are drilled and tapped. (C) Under the intake manifold, the last two digits of the displacement (i.e., 55 for 455, 50 for 350, 00 for 400) are cast into the center of the lifter valley.