Ah yes. I thought for a second there that the F89 chambered in 7.62, then checked wiki and you were right. Jeez, it's been a while.
But yeah, is there a way to differentiate between the two weapons visually fairly effectively?
I don't recall, I just play games

(I love machine gunning in Arma... Mabye I should do machine gunning in the army, but it's not really my thing... I doubt too much). A quick look at the weapons tells me that the MAG (M240, the 7.62 one) has a longer barrel and a slonger stock. But that is not a really effective way.
Best way would just to look at the ammo it contains i'd geuss. But then there's also the Mark 48, which is an M249 retrofitted with 7.62 bullets. AAAARGH
And blergh! All those different names! All those different variants of the same belgium machine gun! why does everyone do that continiously?