First time buying one of these, what should I check before handing over cash?

Found a guy on Facebook Marketplace selling a used water ibc tank for what seems like a fair price, but honestly I have no clue what to look for when I go see it tomorrow. The listing says it only held olive oil before being rinsed out a couple times, which sounds promising because no nasty chemicals, but I'm still nervous about cracks in the cage or the liner being damaged in a way I can't see. Should I try filling it on the spot to check for leaks? That feels like it would be awkward in someone's driveway. Also wondering about the valve threads and whether the cap on top seals properly since I plan to use it for storing rainwater at my allotment. Anyone been burned by a bad secondhand buy and learned some red flags the hard way? Would really appreciate a checklist if you've got one.