Add this to your template just after the body tag whilst developing so you can see where the bootstrap breakpoints are.
Change true to false to turn it off.
<?php if (true) : ?>
<div class="bg-warning p-2 d-block d-sm-none">xs</div>
<div class="bg-success text-white p-2 d-none d-sm-block d-md-none">sm</div>
<div class="bg-danger text-white p-2 d-none d-md-block d-lg-none">md</div>
<div class="bg-info text-white p-2 d-none d-lg-block d-xl-none">lg</div>
<div class="bg-light p-2 d-none d-xl-block">xl</div>
<?php endif; ?>