Brass that has been fired threw a M-249 with a worn barrel/chamber will blow/expand the hell out of the brass. Since the case webbing never expands, the case just above the web budges, and when resized, that area of the case is paper thin, and splits during the next firing. Since the primer may still be staked in on once fired brass, the dead giveaway is when you go to resize the brass, it will take a lot more effort to size back down.
Also, brass that has been worked (read reloaded several time after being fired in a loose chamber) will have been overworked, and just waiting to split. On this type of brass, you get a clue when you go to seat the primer, it will nearly just fall in.
Note: If your running a progressive press, you never get the feed back that you would if you loaded on a single stage press.