Consider the following test case:
If this works the same for you as it does for me, when generated as a tagged PDF, the first list's items will have Lbl elements, and the second list's items will not.
<!DOCTYPE html>
<html lang="en">
<head>
<style>
ul { list-style-type: none; }
li::before { prince-pdf-tag-type: Lbl; }
ul.alphabetic-pseudoelement li::before { content: '●'; }
ul.image-pseudoelement li::before { content: url('data:image/svg+xml;utf-8,<svg xmlns="http://www.w3.org/2000/svg" width="10" height="10"><circle cx="5" cy="5" r="4" /></svg>'); height: 10pt; width: 10pt; prince-alt-text: 'List bullet'; }
</style>
</head>
<body>
<ul class="alphabetic-pseudoelement">
<li>Lorem</li>
<li>ipsum</li>
<li>dolor</li>
<li>sit</li>
<li>amet</li>
</ul>
<ul class="image-pseudoelement">
<li>Lorem</li>
<li>ipsum</li>
<li>dolor</li>
<li>sit</li>
<li>amet</li>
</ul>
</body>
</html>
If this works the same for you as it does for me, when generated as a tagged PDF, the first list's items will have Lbl elements, and the second list's items will not.