If your bolt has a working length of 2 inches a 100,000 psi prestress will lengthen it 0.0067 inches - a small number but easily read on a good micrometer. If the bolt has been properly hardened, such that this load is below yield, it will act like the very stiff spring that it really is, and relax back to it's original length when unloaded, just like any other spring. Many fancy systems are used to properly prestress bolts and studs which are not so easily measured as a rod bolt. Properly designed bolts can be read for length by ultrasound in blind locations. Special load measuring washers are available where an outer ring is free to spin until the design load is reached.
Special bolts are available with a pin indicator built into the head.
No claim is made and no liability is assumed, expressed or implied as to the technical accuracy or safety of the material presented. The viewpoints expressed are not necessarily those of Chapter 1000 or the Experimental Aircraft Association.