Will my customers be able to tell which supplier my flights and hotels actually come from?

Yes in most cases your customer never needs to know which supplier the booking came from, and that is really the point of a white label portal. The search results, the booking pages and the confirmation all carry your own name and your own logo, while the wholesaler sitting behind the connection stays out of sight. What you cannot hide, and should not try to hide, is the airline that actually flies the route or the hotel your customer is staying in, because those are the people your customer is travelling with.

There are still a few places where a supplier name can slip through, so it is worth knowing them. Booking references sometimes carry a supplier prefix. If the supplier collects the money rather than you, their name can show up on your customer's card statement. Hotels quite often send their own confirmation email straight to the guest, and airlines send schedule change notices to whatever contact address sits on the ticket. That is not something a portal can simply switch off, and anyone promising you otherwise is not being straight with you.

The parts you do control are usually enough. Your portal, your emails, your tickets and your hotel vouchers can all be branded as yours, and using your own payment gateway keeps your name on the statement your customer sees. How much of the rest you can change depends on the supplier you are connected to and the plan you are on, so it will not be the same for everyone.

The sensible next step is to make one real test booking on your own portal and read everything your customer would receive, from the payment screen through to the voucher and the emails, then ask about anything that still shows a name that is not yours.