NWA744, you can follow departing aircraft until they're on a high-alt freq-- right? Next step is to keep listening to that high-alt freq until you hear aircraft passing overhead from, say, the West Coast to JFK
. Listen for their readbacks of the freqs they're switching to, until you've got a bunch of high-alt freqs in your area. Hopefully one of them will also have some Detroit arriving traffic, and you'll be able to hear their readbacks of the low-alt freq they're switching to.