If you can get a good close view of your cuttle you should be able to see it breathing. This is easier said than done in a large tank.
When food is offer ad libitum (all the time) which is common for raising hatchlings, it is often very difficult to observe the animals feeding and thus know if they are eating or not. This is a "normal" problem at this stage. You can use smaller individual tanks (there is an article on what I used for octopuses on The Cephalopod Page). Even then, observing feeding is difficult and rare but counting food items and there potential decrease as they are consumed becomes possible.
Sometimes hobbyists continue to provide food all the time for older cephalopods. . . they often rarely see their animals. For hatchling cephalopods, the goal is to keep them alive and growing so providing food all the time is recommended. For older cephalopods, most people would get more enjoyment out of seeing them and therefor benefit from moving away from ad libitum feeding to feeding at a somewhat scheduled time that associates the owner with feeding.
Cephalopods typically learn quickly when live crustaceans are involved!