A few of the 6X6 front diff centres did have trutracs in them. I think they would have come from LRPV's. I have only seen a couple of them and they both had the round pinion drive flanges with fine splines. They used 3.5 trutracs and added a crownwheel spacer to compensate for the smaller diameter 4.7 pinion. Apart from pulling it out the only other way you can tell is by rotating a hub and see which way the other hub turns. In a normal open diff the other hub will rotate the opposite direction - in a trutrac or limited slip it will turn the same direction as the hub you turn.