You can use your Seller Admin > "Send free thank-you email/download link..." to issue yourself a free test email, which should contain a download link that actually activates the redirect if you have one configured.
Note this trick will only work for products with File Download enabled (as that option includes a thank-you/download page link in the email), so if you are issuing a test email for a non-digital product, temporarily enable File Download for that product and Submit to finish editing the product, then issue the test email to yourself for that product, then disable File Download once you have used the link in the email to verify that your thank-you page looks/redirects as you intend.