To check if a specific Checkout UI extension has been installed on a Checkout page, you can navigate to the Checkout area of the admin settings, & click Customize.
Then, use the dropdown list at the top of the page to navigate to the checkout step where you have added your extension. You can look for your extension in the list of installed apps. If it’s not there, then it means the app is not installed.
As for showing a message like "Please enable the Checkout extension first", there isn't a built-in way to do this in Shopify. However, you could add this instruction to the documentation or description of your app to inform your users.
Thank you for your response, Yes I know how to check, I mean I should check this programmatically I just wanted to show the message if it's not installed (because of UI/UX improvement), for example If I show the message always they may think that `I should install it again` or something like that