Weight capacity of a 1-1/4" trailer axle?

I just got a used trailer, it has a 1-1/4" square dropped axle. The seller said it was good for a 3500lb capacity, and that it was a solid core. I’m skeptical of that.

How do I tell what the true capacity is?

I’m hoping that it will at least hold 2500lbs, but I can easily resell it if not.