Magento 2 One Step Checkout module is designed with Auto Detect Customer Email feature. When your shoppers type their emails in the field, One Step Checkout will automatically verify the email if they already exist on your store.
Particularly, if the shoppers have not registered with the input email, the checkout process will continue normally with the Guest checkout mode. In contrast, in case the email has been stored in the system as the customer already used it before, there will be a notification for them to log in right on a pop-up for shortening the checkout process.
To have a closer look at how One Step Checkout extension auto-detects customer emails, please see the short video below.