All storage items have a default value equivalent to zero that is identical whether something is stored as that value or the value does not exist.
With a struct, it is as if the struct already exists, but with all its members set to their defaults. There is no pure way to ask "does this struct exist?".
You can can handle this by giving your struct a field called something like
.exists, and setting it to
true when you create the struct. However, in practice you normally find that there is some field in your struct that always happens to be non-zero if the struct exists, so you can check for that instead of creating a dedicated field for that purpose.