Having a 402 built, how do I know it's really a 402?
 

I mean, I trust the local machine shop that is doing the work. They don't do alot of LS1 stuff, but how do I really know that my 402 is a 402 and they didn't try to save money and just do a 383 or something.

what can I ask as proof of the cubes

Are they installing the motor or just giving it to you to install? Its pretty easy to measure bore diameter and then turn the crank to measure how far the pistons travel in the bore.

well, theres no way its gonna be a 383... if ur giving them a 6.0 block, it will either be a 364 (stock 3.62" stroke), or a 402 (4.00" stroke), unless they bore it 30 over, then u will have either a 369 or 408...