TypeScript check if return type is an array

I am setting up some apis using typescript. I have a particular api that either returns an array of objects or a string.

interface SubmissionDetails {
  branchName: string
  clientAddress1: string
  clientAddress2: string
  clientCity: string
  clientContactEmail: string
  clientContactFullName: string
  clientName: string
  clientState: string
  clientZip: string
  id?: string
  namedInsured: string
  submissionID: string
  underwriterDisplayName: string
}

  getAllReferralsBySubmissionId: (
    submissionId: string
  ) => Promise<SubmissionDetails[] | { message: string }>

Now I am taking the return value from getAllReferralsBySubmissionId and trying to run some logic.

  if (typeof referralsList === "object") {
    referralsList?.forEach((item: SubmissionDetails) => {
      item.id = item.submissionID
    })
  }

I am seeing a TypeScript error on the forEach function stating:

Property 'forEach' does not exist on type 'SubmissionDetails[] | { message: string; }'.
  Property 'forEach' does not exist on type '{ message: string; }'.

I understand that my function can return something that is not an array.
How can I update my function to remove this error?