There are different ways to do it but they all take time and tools. The most accurate way for a home shop to do it is to install the crank and the use one piston and rod assembly that you install in the four corners. That allows you to measure the deck at all four corners with the same rod and piston. That should be good enough to tell you where you are at.

But, the deck almost always needs to be cut or touched up on a Mopar so you might just be duplicating work with what the machine shop is going to do.