PayPal's checkout site normally handles that automatically pretty well. They will normally show the "Don't have a PayPal account?" section to all buyers by default. However, if PayPal's site detects a cookie in the buyer's browser from a previous PayPal account login, then they may assume the buyer already has a PayPal account, so they would show the "Pay with my PayPal account" section in that case.
If you are testing your own cart with a PayPal checkout, you may also encounter the latter behavior since presumably you would have logged into your own PayPal account recently; in this case, you should be able to see PayPal's card-based checkout option by clearing your browser cookies before you test your cart, or by at least deleting cookies for "paypal.com", or by testing with a different browser program that you've never used to log into PayPal.