In most cases the main giveaway will be the vacuum or air pipes at each end.
The monster wagons of the SAR two feet system are very different, however sitting high on their bogies with all the gubbins on show underneath for all the world to see.
That's why when I was designing the kits - with more than a little help from the clever chaps at Narrow Planet - we came up with an etched brass fret which includes the bogies and all the hangers and other bits and pieces which can be cut out, glued into place on the body and the chassis to look something like the brake gear.
It's not an exact mechanical representation, but it looks good enough when the wagons are running on a layout I reckon.
The best bits, in my opinion, are the handbrake wheels, with their distinctive dented rims.
Two of those get soldered onto a bit of wire and fixed onto each end of the wagon.
You can just see them if you look carefully at the left hand side of the picture.
They'll stand out a treat when they're painted yellow against the red oxide colour of the wagon body.