If you have a mic or good caliper, you can measure the width of the threads at the end and work your way up toward the unthreaded part of the stud. The width of the threaded section should be the same along its length on both ends.

You are looking for necking down of the stud, of a narrower section that indicates they have been overstretched. If they are really bad, you can see the narrowed section just by holding them up against a light and looking at them.