One way that I indicate off edges on the mill for deeper features, when the feature in question is too deep for my normal means of finding an edge or axis. With some time spent testing and calibrating your approach with this method, it can help solve the need within the tolerance of many jobs instead of spending money and time buying/waiting for longer edgefinder/coaxial indicator tooling.
This concept can be extended in a variety of ways, one is by using additional features at the end of the rod, like if you need to reach into an undercut then the rod could have a feature turned on the end which extends the reach horizontally but only at the tip