Look at the brass. If you look close, you should be able to see if it's been through a re-sizing die or not.
Factory new ammo will be very straight and clean. Reloaded ammo won't be quite as perfect.
Also look for tarnish on the brass. A lot of the brass I reload has stains and imperfections that don't fully come out in the tumbler - unless they wet tumble, then it gets pretty clean