obviously you know that money is going missing, so you will know when it is going missing, check your rota's see if there are specific staff on everytime it happens, if there is then you can narrow down times when you need to check the CCTV
Also first thing fill the tills with money you have marked, take another amount of notes, mark them and get friends/family or other people you can trust to go and buy items from the store, due to this nearly all of the cash in your till will be marked and you can check your staff as they leave