An image without a proper alt value is usually the worst idea – but of course, it is possible. We can easily debug the missing attribute or the missing value with the following lines:
img:not([alt]), img[alt=""] { outline: 5px solid red; }
Using this code in a live site is not the smartest way, but you can make an own debug toolkit with a bookmarklet – like a11y.css – or load these in a WordPress preview mode which is for only the editors.