fontawesomeでもいいが、単純に文字コードで。

.checked:before{content:'\2714'}
String.fromCharCode(0x2714)

全部

<label class="checked"></label><br>
<label class="a"></label>
<style>.checked:before{content:'\2714'}</style>
<script>document.querySelector('.a').textContent=String.fromCharCode(0x2714)</script>
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