I know what the differences are, but how do you tell the two apart?

Generally the quickest way to tell by sight is to look at the front sight ears.  Inch guns have a hole in the ears, while nearly all metric guns do not.  Isreali guns have a different ear set-up as well.  

On the lower you can tell by the rear of the lower.  On the inch there is a curved portion of metal that extends at the bottom into the stock.  On the metric the rear of the lower is completely vertical.

There's other small differences that let you tell what is what, but those two are the quickest way to tell from a distance or in photos.

Ross