/diy/ - Do It Yourself

Maybe this belongs in /ck/ but I trust /diy/ far more.

I found a pressure cooker among my late grandmather's things, it's very old, likely from the 70's. However, it's almost brand new apart from some oxidized metal and some scratches, the plastic valves etc. are all still in the original plastic packaging inside along with the usage booklet, she likely never used it other than to cook as a regular pot once at most. The plastic ring inside the metal cover used to seal it isn't cracked and feels flexible enough, and it closes properly.

Thing is, I never used a pressure cooker before so I can't actually tell if it's safe condition. What are my chances of this thing blowing a hole in my kitchen ceiling if I start using it regularly? What should I look for in the cooker to be worried about, and how should I test it? Also, what's the worst that could happen? (it's not huge, actually fairly small.)