Though I am one, you don't need an ME to test this. Just do as I did for the rear X-member and diff bolts. Buy a couple, measure them, install ONE using torque procedure in FSM. Then, remove bolt and re-measure and compare with unused one. If it is longer than it started, it has yielded. If not, it hasn't. It's really not complicated.
gorgbroza's post is pretty much spot-on.
There are bolts designed to be installed past their yield point. They almost always have reduced shanks to guarantee where the stretch will happen. With a constant diameter shank, if it stretches it is going to stretch in the threads because the root is the smallest diameter. The amount of stretch in the threads is something less than consistent and reliable. The stretch also happens along a much shorter length of shank which decreases the fatigue life. With a reduced shank bolt, the shank is a smaller diameter than the thread root and therefore has less cross-sectional area. When tightened, this smaller CSA is where the stretch occurs. It also occurs over a longer length of shank and increases fatigue life compared to having the stretch happen in the threads.