How can you tell ?

I pulled one off a 87 IROC, 5.0 TPI, it measures 1 11/32", feels too light to be solid, tho I can't weigh it.

The ends do seem to have a parting line, as if a tube had been pressed together, but I'm thinking this could also be from the shear used to cut a solid rod.

Are there any other visual clues?

 

I believe there's supposed to be a drain hole (somewhere near the end link I think) on the hollow ones. It's probably covered up with dirt.
If it feels light, it's gotta be hollow. I pulled the solid one off my 82 Z28, it was really heavy.
The one I pulled off a 87 GTA was hollow and a LOT lighter. It measured 1 7/16" at the middle. (36mm??)

 

Thanks, i found the holes I'd missed, probed them with a scribe, it's definately hollow.