From what I know, adding a credit card doesn't actually change the account from unverified to verified. You need to add a bank account. To see if someone added a bank account, fill out all the information to send somebody a payment. Once you click continue, (on the confirmation page) it will say whether the account is verified or unverified. Unverified would state they haven't added a bank account.
Accounts registered under a USA address are prohibited to send money until a bank account or credit card is added. A workaround is registering a Paypal under a fake address from the country of Canada. This will give you a $3000 sending and receiving limit, but these unverified account are often limited for suspicious activity so watch yourself.
Go to Paypal.com > Login > Merchant Services > 'Buy Now' Button. Here you can customize your button. Step 3 includes a checkbox for "Take customers to this URL when they finish checkout".