The casting numbers would always tell you for sure what it is.
But, if you're junkyard hunting, there are a few identifiers. The 2.8 was 60*, so the block will be narrow with the heads close together. The Buick V6s will have the dist at the front of the block. The difference between the 4.3 and earlier Chevy V6s will be the dist. The 4.3 is even fire, so the dist cap will have wires evenly spaced. The 3.3/3.8 have uneven spacing of the caps.
