I`m not sure if there`s a way to confirm it without pulling the pan, but I always look in suburbans and 4wd`s, or any truck application that had a factory 8 inch balancer, every one I ever seen with the 8 inch had 4 bolt main caps, vans are also a good place to look. The later Vortec blocks can have 8 inch balancers with 2 bolt caps, but the generation 1 small blocks were all 4 bolts that I`ve seen thus far.
If you can get to see the casting number (over the oil filter housing, topside of the bellhousing),you can be 95% sure that a #3970010 will have 4 bolt mains. Hope that helps. All of them I've seen with that # have been 4 bolts anyway.