How long has it been in there? Is it still good to use or will there be a science experiment inside?
My go-to solution is the good ole sharpie. Label the lids on the jars with the date they were opened before putting them into the fridge.
Use a sharpie to label leftovers, takeout boxes, ziplock bags, food wrapped in aluminum foil, freezer bags, and on Saran Wrap too.
Never be afraid of your leftovers again!