One quickie way is that the perch bolt bosses are slightly recessed in the web of the I-beam due to the increased thickness of the web.
I think someone did some compare and contrast on all the axles. Even though they look stout, they aren't that much heavier. I'm thinking +/- 2 lbs.
it's not that they are weigh more that they are called "heavy" axles. it's that they have a bigger I-beam between the spring perch and the kingpin boss. makes the axle look weird not dropped but PERFECT when you drop it.