I've always liked ultrasonic bolt length measurement as the ultra high reliability way of measuring preload.
It can also be used for confirming the correct preload is still present, with the downside of having to track the original length of every fastener (which isn't really a big deal in high reliability applications)
I consult with a seismic expert at UC Berkeley about once yearly and he's pretty cavalier about pre-loading ...
Which is to say, he wants me to "pre-load the shit out of it".
Which is to say, our "indicator" is that we start to bend the HDB with the threaded rod. For instance, if we have two of these, horizontally, on either end of threaded rod:
It can also be used for confirming the correct preload is still present, with the downside of having to track the original length of every fastener (which isn't really a big deal in high reliability applications)