You believe you have an AMC304 V8. I take it you have no reason to believe it is not a V8
so is the question about checking the capacity it was made with or the capacity it is now?
I believe, and someone can correct me if wrong, that it is on the side of the block near the engine mount, says 304 or 360 or whatever. From 304 to 401 they all were made from the same block casting, there is no small or big block in an AMC (too tight fisted maybe).
The engine number is on the front top right near the end of the valve cover. I beleive this is a sequenetial number.
If you can find the engine identification it should be something like 212H05 The 2 would be 1982, the 12 would be December and H a 304ci. the 05 would be the 5th day of the month.
If you want to check the VIN it should start something like IJCHM>>>>>>. The H is for the 304.
If you check all this and you are finding that H in the engine serial number than you had a 304 originally.
If you REALLY want to know then you will have to take the head off and measure.