How can I test if a curried function was called?

// builder.js
class Builder {
  build = (value) => () => value.trim().toLowerCase()
}

export default Builder

// builder.test.js
import Builder from './builder.js'

describe('Builder', () => {
  it('should build trimmed lowercase string', () => {
    const buildInner = jest.fn()
    const buildOuter = () => buildInner

    expect(buildInner).toHaveBeenCalled()
  })
})