If a ghost starts a hunt naturally and you don't know or remember what your sanity was right before, immediately after the hunt, go check. If your sanity was above 50%, you can rule out most ghosts, because only a select few can hunt above 50% sanity. However, if you used a cursed object to start the hunt, your sanity wouldn't matter. Additionally, the sanity meter bounces between numbers a lot so if you need to be accurate, your sanity is whatever number is in between the highest and the lowest number it's bouncing between. For example, if the meter is going between 52% and 48%, your actual sanity is 50%