Someone told me this years ago, I cannot say how valid it is but FWIW.
Put the old ammo in an air tight container for a couple of days, then upon immediately opening the container smell it, if you get a faint vinegary odor then toss the whole lot, the powder has gone unstable.
Other than that, just check the cases for signs of corrosion and/or damage and toss the suspect ones.
People have been shooting 80 year old milsurp safely for a lot of years.
FWIW.