A 3500# axle usually has 5 bolt hubs and I think a 2.5-3" tube. 5200,6000+ will have 6 or 8 bolt hubs and 3.5" tube.
Look for marks, tags, labels, stamps. Also look at e-trailer or Dexter and compare dimensions.
There are so many different brake sizes,How do you measure the brakes to see what size you have, with the brake shoe on a have moon shape, just follow the shape and mesaure or from one end to the other and width? Told ya Im a newbie!!