You usually don't find out until the magazine comes out, at least that's been my experience
It depends on the magazine but they'll either send you an e-mail, a postcard or a copy of the magazine. (At least you hope they do, I'm in the current issue of Somerset Studio and they still haven't contacted me to tell me, I had to buy a copy for myself.) I usually submit for specific calls and so I know when that issue is coming and I keep an eye out for it. Also they'll keep your items for a long time, most 6 month to a year but I also had a few things kept for two years before they were returned.