There are a few ways to check if a domain is available on Shopify. The first way is to use a Shopifydomain checker tool. This will allow you to enter a domain name and see if it is available on Shopify.
Another way to check if a domain is available on Shopify is to contact Shopify support. They will be able to check the availability of a domain for you.
Finally, you can also check the availability of a domain on Shopify yourself. To do this, you will need to log in to your Shopify account and go to the "Domains" section. Here, you will be able to see a list of all the domains that are available on Shopify.