One (relatively) easy way to identify an A-series engine vs. an A+ engine is by taking a look at the distributor.
The A-Series Engine
An A-series distributor has a clamp around it, which has a horizontal pinch-bolt. This clamp is held to the engine with two bolts.
The A+ Engine
The A+ distributor is held to the block with a forked mount. One bolt secures this to the engine block.
If still in doubt, please contact one of our classic Mini experts :)