I straighten it out completely, put a small 90 degree hook on one end, sharpen it on the inside edge and then run it in to the casing base. When withdrawing, you will feel a dip where the case wall expanded to the chamber's wall. If the sharp hook catches on the forward side of the dip, you have potential incepient separation.
The best indicator is when you can visually see a thin, bright shiny ring in the same area on the outside of the case. When first starting, it doesn't go all the way around the case.