After assessing your ammo per above instruction, and then deciding to shoot it anyway, be prepared for a much higher charge from the powder.
I went through two boxes of 12g a few months back that were very old. Paper-wrapped old. Handed to me from my grandfather old. 59 cent price tag on the box old. You get the idea.
Anyway, to make a short story long, this stuff knocked my fillings out, detached my retinas, and now I can't remember my own name. 2 3/4 inch standard issue bird shot that felt like 3 1/2 inch magnums.
Of course, being from the desert, and having boxes of ammo sitting in a 130+ degree garage drying out for 25 years may have contributed greatly to this. Depending on where you are, your mileage may vary....