Can I test how my white label website works before I put it in front of real customers?

Yes. You can run your site in a test mode first, where the search, the booking form and the confirmation emails all behave exactly as they will on the live site, but nothing real happens at the end of it. This is the normal way to open a portal, and you should plan for it rather than going straight to live on the first day.

How far the test goes depends on the supplier. Many suppliers give a test set of credentials that returns made up flights or hotels, so you can book right through to a confirmation number that is not a real reservation. Some suppliers do not offer that at all, and in those cases the only true test is a small real booking that you make yourself and then cancel inside the free cancellation window. Payments work the same way, most gateways have a test mode with card numbers that never charge anybody, and the ones that do not will need one small live payment to prove the money lands where you expect.

The things worth checking in that period are the ones customers notice first, whether the prices on the results page match the payment page, whether the confirmation email carries your brand and not somebody else's, whether the site reads properly on a phone, and whether a booking made by a customer actually shows up in your admin panel.

Ask which of your suppliers give test credentials before you sign with them, and set aside a week to book, cancel and refund a few dummy trips end to end so nothing surprises you on the day you open.