JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li> 
        <a>Top Level with Sublist</a>
        <ul>
            <li> 
                <a>Sublist</a>
            </li>
        </ul>
    </li>
    <li> 
        <a>Top Level with no Sublist</a>
    </li>
    <li> 
        <a>Another one with a Sublist</a>
        <ul>
            <li>
                <a>Another Sublist</a>
            </li>
        </ul>
    </li>
</ul>

CSS

li > :first-child + ul:before {
    border: 5px solid transparent;
    border-left: 8px solid #522;
    width: 0;
    height: 0;
    content:"";
    display: inline-block;
    transform: rotate(90deg);
    margin-right: 5px;
}