The stamped steel tag on the backside near the dipstick port has this. If you Google this exact question with "4t65e", you'll find a handful of site that have tons of charts on this.
If that tag is gone, it's not so simple. You can tell if it has an HD differential or not by comparing to pictures of the 2 types, and maybe see what torque converter bolt pattern diameter it has (245 or 258mm). Those don't tell the whole story, but it's something to start with before digging inside.
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