Will my customers see the local city tax or resort fee before they book a hotel, or do they only find out at check in?

In most cases yes, they will see it before they book, as long as the hotel supplier sends that information along with the rate. Hotel booking APIs normally split the cost into what the guest pays you now and what they must pay at the property, and the portal shows that second part as a separate line in the room details and again on the review page before payment. It usually appears with a label such as city tax, tourist tax, resort fee or cleaning fee, so your customer can see it is extra rather than hidden inside the room price.

The honest limit is that this depends entirely on the supplier data. Some suppliers pass mandatory extras as proper structured amounts, which we can display cleanly. Others only mention them inside the long hotel policy text, and a few say nothing at all until the guest arrives. New local taxes are also a common gap, because when a city introduces one it can take a while to reach supplier feeds. So the portal shows what it is given, and we cannot invent a figure that the supplier has not sent.

Because of that, the safest habit is to treat these extras as something you mention rather than something the system guarantees. Many agencies add a short line to their booking terms saying that local taxes and property fees may be collected directly by the hotel, which protects you when a supplier has not supplied the detail.

What I would do next is run a few test searches in the destinations you sell most, open the room details and the confirmation voucher, and see how clearly the payable at hotel section appears for your suppliers, then tell us which ones look thin so we can check whether more of that data can be pulled through.