Theres only one way to tell. All the way in the back of the motor, behind and below the drivers side valve cover, youll see cast in the block "5.0" or "5.7". 5.7 is 350, 5.0 is 305. It will be difficult to see, but if you can get to it, use a flashlight.

 

thanks for the advice, i'm thinking its a 305 now and the guy just lied to me. I just did a carfax report. I'm pretty angry about this, but i got the car 5 years ago, so there is pretty much nothing i can do.