If you pull a spark plug, stick a dowel into the cylinder and turn the crank, measure the stroke, the difference between the highest point and lowest point. If it's 4" it's a 400, if it's less, it's a 351M. That is, as long as the engine block is a 400/351M engine block and someone didn't swap in something different before you got it. Plus, if the valve covers are original, then one of them should have a label on it saying what it is.