This is how vue devtool detect vue js. Just paste this snippet on the console.

Vue Js detector on a site

const allDom = document.querySelectorAll('*')
let el
for (let i = 0; i < all.length; i++) {
  if (allDom[i].__vue__) {
    el = allDom[i]
    console.log(el, "This is the vue element")
    break
  }
}
if (el) {
  console.log('Vue exists!')
} else {
  console.log('Vue does not exist !')
}