There is an easily seen casting difference on the front of the block on the passenger side.
If your finger will lay in the slot that's formed by the casting ridge, it's a later 265 or 283. If a finger won't fit, it's going to be a 4" bore.
Otherwise just get the suffix code from the pad on the passenger side front of the engine. It will look like TO5I4TL as an example. There may be 1, 2 or 3 letters at the end. The suffix will say what the engine came in originally.
At the back of the block will be cast in numbers that will help tell what engine it is as well, along w/a date code.