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There is a service port on the side of the tranny that most guys use.
I've seen a chart floating around gmfs that gives you the expected miles out of a tranny vs. temp. Basically said that if you keep it under 140 deg (daily) it'll last forever, and if you run much over 200 deg (daily) you'll be lucky to get 20k miles.
The tranny temp in our 3500 work truck never gets over 190* towing a 10K lbs trailer (max towing load per owners manual) and never gets over 140* unloaded (tools and guys, but no trailer). It's got 135,000 miles with no tranny issues yet and it's towed at least half those miles.