I have never modified an LS, but from old experience with other cars, I would say this engine has a hotter cam in it and louder mufflers.
Don't think you could tell if an engine has been stroked just by the sound, all other things being the same, unlikely. An engine that has only been stroked doesn't sound much different from stock. But rarely do you leave anything else in the engine stock if you went thru the trouble of stroking it.
Unless this guy pops the hood or shows time slips it would be hard to say more than cam and mufflers.
Just my opinion,
Jim Henderson