TLDR;
The kind of type T is interface if its underlying type is an interface type.
The kind of type T is struct if its underlying type is a struct type.
Spec: Struct types and Spec: Interface types specifies exactly what are the struct and interface types:
StructType = "struct" "{" { FieldDecl ";" } "}" .
FieldDecl = (IdentifierList Type | EmbeddedField) [ Tag ] .
EmbeddedField = [ "*" ] TypeName .
Tag = string_lit .
InterfaceType = "interface" "{" { MethodSpec ";" } "}" .
MethodSpec = MethodName Signature | InterfaceTypeName .
MethodName = identifier .
InterfaceTypeName = TypeName .
So for example these are struct types:
struct { A int }
struct {}
struct { _ int }
and these are interface types:
interface { String() string }
interface {}
We may use a type declaration to create a new type, such as:
type Point struct { X, Y int }
The above type definition creates a new, distinct type with the same underlying type and operations as the given type, and binds an identifier to it. The definition of underlying type is recursive:
Each type T has an underlying type: If T is one of the predeclared boolean, numeric, or string types, or a type literal, the corresponding underlying type is T itself. Otherwise, T's underlying type is the underlying type of the type to which T refers in its type declaration.
When we talk about arbitrary types being structs or interfaces, we're talking about their kind.
In the light of this, basically your question is equivalent to this:
"When is the kind of an arbitrary type interface or struct?"
The answer to this question is not in the spec, but this is how we could define it:
The kind of a type T is interface if its underlying type is an interface type.
Similarly:
The kind of a type T is struct if its underlying type is a struct type.
So for example:
type Point struct { X, Y int }
type PP Point
Is the type struct { X, Y int } of kind struct? Yes, because since it's a type literal, its underlying type is itself, and it's by definition a struct type.
Is Point a struct? Since the underlying type of Point is the underlying type of the type to which it refers in its type declaration, which is a type literal (see above), it is of struct type (its kind is struct).
Is PP a struct? Since its underlying type is the underlying type of the type to which it refers in its type declaration (which is Point), whose underlying type is a struct type literal, yes, it is also a struct type.
This kind we're talking about is represented by the reflect.Kind type. There are reflect.Interface and reflect.Struct constants (of type reflect.Kind) to represent the struct and interface kinds. And the reflect.Type type descriptor has a Type.Kind() method to access this kind.
This is how you can check if the type (kind) of some value is a struct for example:
func isStruct(i interface{}) bool {
return reflect.TypeOf(i).Kind() == reflect.Struct
}
Testing it (try it on the Go Playground):
fmt.Println(isStruct(Point{})) // true
fmt.Println(isStruct(PP{})) // true
fmt.Println(isStruct(struct{}{})) // true
fmt.Println(isStruct("text")) // false
Checking for interface type is a little more complicated because passing an interface value to a function that expects interface{} will not pass the interface value as-is but the concrete value "stored" in it, and as an interface{} value. We would have to pass a pointer to interface (which otherwise rarely makes sense in Go), access the element type and check its kind. For details, see this answer: What is the difference between reflect.ValueOf() and Value.Elem() in go?
