Checking external private errors

Question

I have a package which does some error checking for specific error types. One of the errors is from an external package, its fields are all unexported as well as the relevant creation func, so I cant instantiate it in my test, like below.

type Error struct {
    message string
    condition bool
}

func (e *Error) IsCondition() bool {
    return e.condition
}

func NewError(message string) *Error {
    return &Error{message: message}
}

func newErrorWithCondition(message string, condition bool) *Error {
    return &Error{message: message, condition: condition}
}

I want to implement a test in my package that checks if the error has the condition set or, as a different path is executed in each case but have no way of instantiating it? Is there a way I can test this?

Im testing something thats like,

func HandleErr(err error) error {
    var specificError Error

    switch {
    case errors.As(err, &specificError):
        return something
    default:
        return another thing
    }
}