JSFiddle - React, Tailwind, and code Playground

HTML

<input type="radio" name="rdo" id="rdo1">Radio1<br>
<input type="radio" name="rdo" id="rdo2" checked>Radio2<br>
<input type="radio" name="rdo" id="rdo3">Radio3<br>

JavaScript

if(document.getElementById("rdo2").checked){
	alert("Radio2 marcado");
}