Not sure if this is exactly what you are looking for but I use this to check if visitors to my page are Coil Subscribers, this is a blanket check and I don’t think you can check who the user is, just if they are a Coil Subscriber or not.
I show various Coil subscription checks on my Coil showcase page here: https://xrp4u.com/what-is-xrp/xrp-ecosystem/coil/
I am am just a massive Coil fan.
<div style=”display:none;”>
<script src=”https://cdn.coil.com/if-monetized.js”></script>
<!– you can put whatever code you want after the script tag –>
<p>XRP4U have detected your Coil subscription! you are amazing! How did I know?
We have not enabled Ads on our site because we do not want to spoil your viewing experience!</p>
</div>