Is there any way to tell if a steering gear box from a JY may be bad before buying it and installing it?

 

Somewhat.
If the preload adjustment screw on top is screwed down into the box, pass it up.
Turn the input shaft and make sure:
1) the output shaft moves accordingly with no slop or lost motion,
2) the input shaft does not move in and out of the box as it's turned back and forth,
3) you don't feel any binding as the box is turned through center (it should tighten up very slightly on center though)
4) no sign of leaking at the pittman seal or anywhere else

 

Great! Thank you