1) Age. Most aren't warranted past 5-10 years 2) how was it handled? Was it in a climate-controlled area up off the ground, out in a shed, or in the back of a pickup? 3) if it has nitrogen in it, is there frost around the neck? Unfortunately most cracks you can't see, and you won't know if it will hold nitrogen until you fill it and let it sit. In my opinion, not worth the trouble. When you consider what you invest in semen and shipping, it only takes one bad tank to blow a lot of money. If you get a tank, used or new, check it often as even new tanks can fail.