You get used to the feel of the drill and can tell when it starts to struggle because it is getting blocked up. Also keep an eye on the amount of waste coming out of the hole if this stops or reduces greatly remove and clean the drill. If you are doing several blanks make sure the drill doesn’t get too hot otherwise this can cause problems. If I am doing a number of blanks I usually have more than one drill so I can swap them around when they get hot.
With acrylics be careful otherwise you could end up with the drill stuck in the blank as the picture below shows. This has only happened to me once but it was very annoying as it was a brand new drill
It is also a good idea to measure the blank against the drill and marking the drill so you know when you are getting near the end of the blank. It is a good idea to remove the drill and clean it just before reaching the end of the blank, then drilling very slowly as this helps to prevent chipping around the exit hole.