The transition from the flat face of the spokes to the outer rim. If it's a radius, it's forged. If it's got the crease that's plainly evident on the lighter-painted wheels, they're cast. The forged wheels were '80 and early '81 only, and always had grey in the painted areas; if the painted areas were keyed to body color originally the wheels are cast. There are also non-color-keyed cast wheels with the same grey as used on the forged ones. I have not yet seen an exception to the paint rule, but that doesn't mean one doesn't exist. Production use of cast & forged has some overlap, so if it's an '81 wheel you have to look for the crease. All the '82-'83 wheels will have it. Were any later cars built with forged? Probably... I have a 340 block from a late-production '73 Duster 340 (May) with a casting date of 9-28-71. They used what they had lying around, regardless of what it might be. This is also why the '74 E-bodies used the '70-style shifter for the console automatics (the one with the woodgrain knob rather than the much cooler T-handle).
The forged wheels are marginally lighter and much stronger, but there's absolutely no reason to avoid the cast--they're plenty strong. In fact, unless you're really going to beat your car on a scale that would scare the producers of "The Dukes of Hazzard" you'll never notice a difference. For that reason, I personally prefer the cast wheels, because I really like the look of that sharp corner crease.