A simple test for deformation would to measure thread pitch over the entire length of the bolt to determine if it has permanently deformed. e.g. a 8 tpi thread pitch should measure 48 threads over 6 inches.
A more complicated method is to look for internal cracks/flaws with an ultrasonic device.
You have said nothing about the application of the bolts and whether or not they needed to be pretorqued. I have seen a terrible accident from bolts that were repeatedly hand tightened over the years of operation and ultimately failing with serious consequences to the operator. So DLite30 first comment is highly appropriate.
Consider the application. If the connections are highly critical or could cause injury or worse if they fail, toss them and buy new ones.
Start by confirmming that bolt/stud is OEM, and not a counterfeit. This means acquiring the original paperwork for the stud/bolt. If you can't confirm this...disgard.
If you can confirm the stud is OEM, visually inspect the bolt for signs of wear/corrosion. Toss if the corrosion has reduced the shank CSA to any visually appreciable amount.
Follow DLiteE30's recommendations above for stud measurement
Perform straight beam ultrasonics on the studs. Discard if any indications >10%FSH are found. Note: Seek out a good Level III technician as the bolt configuration may be such that a poorly experienced technician cannot differentiate geometric configurations from cracking. Test from both ends; you may have to grind one or both ends flat for good contact.
Consider wet fluorescent MPI if the condition of the bolt/stud is conducive to the test.
I would avoid PT as a greater level of cleanliness is required than for MPI. Its generally difficult to remove enough PT from the threads, and overcleaning will probably occur.
Having said that, if you want to try PT, avoid the solvent removable method as there is too much drama involved with the cleaning process (this is the same liguid found in dye packs used in banks).
At the end of the day, MPI is a superior method for this application, and coupled with straigh beam ultrasonics, will provide the best detection of cracking.
One final note, a variant of phased array ultrasonics would provide the highest level of detection, but would be cost prohibitive--by the time you get the calibration standards (effectively the same studs you will be testing) and perform the test, the cost to replace the studs will be less than the testing involved. And, after all that, you still have used studs that will probably not last as long as you want them to...
I don't recommend dye penetration NDE be considered in this application: Since the highest stresses (most likely fail points) are at the smallest bolt/stud cross-section, the highest stresses are at the root of the thread, not the smooth shaft of the bolt/stud.
You can't adequately clean the thread's root of a used bolt, but can only (realistically anyway) clean the smooth shaft. So the dye pen test will only show "good metal" where the stress is least. You'll always get dye pen residue (implying a crack) just where you want to look.
An NDE person's time (travel plus wages, plus cleaning time of each bolt) is worth more than dozens of new studs.
Easiest way to evaluate unacceptable 'stretch' is to get a 'good*' nut and see if it will screw smoothly onto each stud, using just a couple of fingers.
* good = a nut that fits snugly onto the used studs. There is a LOT of difference in the closeness of stud-to-nut fit from mfr to mfr, and lot to lot. And they are all within industry tolerances.
