tracker/installation_support/check-gtm.js

const GTM_SIGNATURES = ['googletagmanager.com/gtm.js']

function scanGTM(html) {
  if (typeof html === 'string') {
    return GTM_SIGNATURES.some((signature) => {
      return html.includes(signature)
    })
  }

  return false
}

export function checkGTM(document) {
  if (typeof document === 'object') {
    return scanGTM(document.documentElement?.outerHTML)
  }

  return false
}