The 302 and 351 can be told apart at a glance.
With a "calibrated eye' the engine is substantially wider, most notably the intake is wider.
Even without the eye the timing cover water ports on the 302/5.0 is right at the head gasket/deck while there is what, 1 1/8? 1 3/8 inches of block between the water passages and deck.
Far as telling one Chevy from another, I can't figure why I would care.
With the F-Body cars, the 350 motor came available in the IROC-Z, Z/28, Trans Am, and Formula 350 cars as the "L98" motor, a 350ci TPI (Tuned Port Injection) motor. This motor was essentially the same as what was used in the Corvette, except the 'Vette got aluminum heads and better exhaust manifolds (IIRC) and that car weighed a bit less than the F-Body cars. Stock rating was somewhere around 230-235hp at the flywheel (the higher rating being for later models that had dual catalytic converters).
This is NOT, however, the same 350 motor that was to be found in your average full-size Chevy pickup truck. Different heads/cam/intake in those, and the truck motors were either carb'ed (in the early 80's) or TBI motors (Throttle Body Injection, same as Ford's CFI setup, in the late-80's trucks), both of which were made mainly for torque and not for horsepower - they were rated somewhere in the ballpark of 180 horses. The later "Vortec" head 350's make for some REALLY nice power, though.
Oh, and extra trivia tidbit: the Chevy 4.3L V6 is basically just a 350 with two cylinders chopped off. It's not a horsepower motor, either, but it's definitely a torque monster, as far as V6's go, and they last damn near forever.
The 350 and 305 look pretty much identical on the outside, which makes it a handy swap if you live in a smog state because unless they know what to look for and if your setup is in good running order, they'll be none the wiser. Thing is, there's been so damned many different versions of 350 motors over the years that just because he has a 350 in there now doesn't necessarily mean it runs better than it did with the stock 305. In fact, the TPI 305 was rated at like 205-215hp factory, which is better than the old 350 that came in the 1981Camaro Z/28 (which was only rated at 180hp).
Unless this kid with the 350-swapped Camaro has an actual BUILT 350, or at least has some bolt-ons to go with it, then chances are he's barely got an advantage over his stock 305. The stock motor for a V8 Camaro RS was the TBI 305, which only had about 170hp. If he kept the TBI setup and just dropped in a 350 TBI motor, he's probably only somewhere around 180 to 190hp; if he went carb'ed and has it even halfway tuned right, I'd guess he's in the ballpark of 200-225hp. In either case, you'd still have him beat on the power-to-weight factor because an F-body has 200 to 400 lbs. more weight than your average Fox.
HOWEVER ... if he's running a decent H/C/I combo 350, then you might be in for a hurting. The average 350 motor has a HUGE aftermarket available, so there's a lot you can do with them, and they've been around for decades. It doesn't take a big budget to build one up to make some pretty nice numbers.
(NOTE: All horsepower figures mentioned above are FLYWHEEL, not REAR-WHEEL figures.)