1990 300SE Muffler part #

I just had the center muffler replaced on my 1990 300SE by a local mechanic.. The car doesn't sound as quiet as it did with the previous muffler- it kind of sings a note now. I'm wondering if I can check to make sure the part # is correct for that car. The muffler he installed is part #126 490 52 15 and it was $364.00 before installation. Also, how can I tell if he used a genuine MB part?Thanks!