How to check that a member function is generated by compiler?

[Andrey Zherikov via Digitalmars-d-learn]

This code
```d
import std.sumtype: SumType;

struct A
{
    SumType!int b;
}
static foreach(sym; __traits(allMembers, A))
    pragma(msg,sym);
```
prints
```
b
opAssign
```

How can I check that `opAssign` is generated by compiler and 
doesn't exist in the original code?