No way to get close without pictures and the VIN number. The VIN will identify what the engine had when it came from the factory. There are other clues on how to tell what a motor is, but I cannot remember them off the top of my head to repeat here. If you do a search, you will find some great hints on how to tell what you are looking at.
[edit] search with retsin, two mints in one.
2 bolts the engine is a: 239/256/272/292/312 (produced from 1954 to 1964)
5 bolts the engine is a: 332/352/360/361/390/391/406/410/427/428/ (FE engine)
6 bolts the engine is a: 221/260/289/302/351W
7 bolts the engine is a: 429/460
8 bolts the engine is a: 351C/351M/400
There are other ways of identifing the difference between a 302/351W, and a 351C/351M/400.
The 351W and 351M/400 both have wider intake areas than their smaller brothers. I don't remember an exact number, but I do know they are about 2" wider. This is because the motors have a taller deck height to accomodate a longer stroke.
*most* 2 barrel Clevland heads(also used on the 351M/400) will have a 2 cast into the upper corner, right below the valve cover sealing area, 4 barrel heads will have a #4 cast in the same position.
A 351W will also have a rib on the block between the top of the belhousing (top two bolts) and the lower intake sealing surface.
More help on narrowing things down. A 351C will have a sub 5" distance(4-7/8"?) between the top bell housing bolts, same as the late 302/351W. The 351M/400 will have about 7" between the top bolts, just like a 460. NO, that does not make that motor (351M/400) a big block, it only shares the pattern, and a few parts with the 429/460.
The 400 started production in '71. It looks alot like a clevland, because the design was copied. They changed a few internals, and used the same style head as a 351C.
It is difficult to tell the difference between a 360 and 390. The only way to tell is to measure the stroke. The 360 had a 3.5" stroke, the 390 around 3.75" stroke. The bore size is the same.
The 351C production ended in '74, a 400 was destroked to 351 to take its place (4" to 3.5"). The 351M/400 was terminated in '79 in cars, and 82 in trucks.
Certain 351M/400 cast before 1977 had problems with cracking in the lifter valley. I dont have the casting number handy, but it was only blocks cast at a certain foundry that are affected. If anyone cares, I will look it up.
Certain rare '72 400 blocks have both belhousing patterns, for big blocks, and small blocks.
429 and 460s, no way to tell from the outside on what you have, have to measure the stroke as well.