Queen bees are larger and can even have a different color abdomen than her offspring, so why are they hard to locate in a hive? Whether the hive only has a few frames of bees, like a nuc, or is multiple stories high with several boxes and thousands of bees, can make the task more or less challenging.
The younger queens tend to be more skittish and harder to find than older queens that are used to our occasional presence. Aggressive colonies, like Africanized bees, which we can end up with in our area (southern states) when a colony "makes their own queen", tend to be very illusive and can be difficult to find, even for a seasoned beekeeper.
The process of elimination using a Queen Excluder as a "strainer device" works well, but requires understanding how the process works and having and extra empty box available to hold frames as they are removed during the process. (... more to come...)