Inspect the checked property of the element.

Say you have this checkbox:

<input type="checkbox" class="checkbox" />

You can see if it’s checked using

document.querySelector('.checkbox').checked

You can also check if looking for .checkbox:checked does not return null:

document.querySelector('.checkbox:checked') !== null

but I think looking for .checked is cleaner.

Do NOT use getAttribute() looking for the checked attribute value, because that’s always true if the checkbox is checked by default in this way:

<input type="checkbox" checked />

Also don’t check for the value of a checkbox element. It’s always on, regardless whether the checkbox is checked or not.