If you are selling download products, you can test that your files will be delivered properly by using your Seller Admin > "Send free thank-you email/download link" to send yourself a link for each download product you have; if you can download and open a file by that method, then your files will be deliverable.
Now, as for testing that your checkout process is setup correctly, either run through a checkout yourself (if you have another PayPal account to send yourself payment) or, as swoodproducts suggested, have a friend/relative/colleague perform a test checkout.