I always look at the silicone seams on used tanks once I get them home. I use a very bright light and check that there is no missing or lifted edges anywhere. If they look as good as a brand new tank, I still leak test the tank for a week but make sure the tank is on a flat, level and secure surface.
I have yet to buy a used tank that met all these requirements, so I strip the interior silicone, remove any residual silicone skin, clean thoroughly, prep all sealing surfaces with denatured alcohol or acetone, let dry and caulk. I prefer to let the silicone cure for 1 week and then water test for an additional week. I haven't had a single failure using this method, so I'm comfortable with doing it this way.