Each of the three angle irons provide a place for the neck blank to rest on with a gap between them. You can look at how the neck sits on each of these rests and see if the neck is completely straight on the resting side or whether there is a slight bow somewhere that needs to be taken care of. You can also see the gap produced by the neck blank and the wood underneath in the "gaps", and measure this distance to check consistency?
cheers,
Gav.