I will tell you the ideal way to check things.Surface plate with height gauge/with indicator in it.Set gauge for height on part for one side,then rotate ,compare measurement,repeat.You can apply what I just said in the mill by using indicator in drill chuck and collet holder on mill bed.
I manual machine about the majority of my parts.That being said,if you do much of any kind of collet indexing,you need to get a 5c collet indexer.It is a must if manual machining much.You will never use those collet blocks again.