Just look it over and be sure it's not junk to begin with.

If the owner doesn't know how many times it's been fired, pass.
Shot out of an M2, might pass.
Corrosion eating into the brass, pass.
Brass with pink or red hues, pass.

That's not true and I would never tell anyone that!

The M2/3 if improperly headspaced can stretch brass beyond useful in one firing! Sure it can be reloaded but if stretched to much, it will fail! When it fails in your firearm, you and your firearm could pay the price for being cheap and or stupid.

You need to know how to check the brass, this means that you'll need to make a tool out of a piece of metal rod. Using a metal coat hanger is ideal for doing this. Cut the bottom straight piece out of the hanger, grind one end to a point and bend it at a 90° small enough to fit into a piece of .50 brass. Make it about 6" in length so that it.will go deep enough into the brass.

You need to drag it up and down the side of the brass about 3/4" from the bottom of the case. If you feel an indentation on the inside of the case, it's bad. That means the case has been stretched and could have a case head separation the next time its fired. Sometimes you can look down in the case with a flashlight and see the ring around the case and if you can see it, that's not good.

Be safe, if you have any questions ask.