As has been mentioned, look for the main cap crossbolts on each side of the block above the pan rail where the three center mains reside. With these in place it’s very likely a 427 although a number of people have retrofitted the four bolt mains to the other FE’s, myself included.
427’s came through as center-oilers well into1965 and sideoilers were available from 1966 and on so it could be either oiling system depending upon the year model of the block. For the 427 sideoiler version, look for the screw in freeze plugs whereas the 427 center-oilers came equipped with standard pressed in expansion plugs. The sideoiler also has the oil gallery in the drivers side of the block just below the screw in freeze plugs along with a galley plug at the very front of the block just in front of the oil filter adapter.
Many of the industrial 427’s were two bolt mains so even though the block doesn’t have four bolt mains, it can still potentially be a 427 without the crossbolts on the outside of the block. Looking for ‘HP’ on the side or back of block can also point you towards a 427 but 406’s also had these markings. And don’t be misled by those 390’s that have the sideoiler oil galley cast in the side of the block and the sideoiler oil relief valve provision in the back of the block while also having ’66-427’ cast into the back cylinder of the block. Casting numbers from the block can also help to identify the block origins. Beyond this, it will almost take a teardown to confirm which engine you have.