Will my hotel booking API tell me the cancellation deadline and any charges before my customer books?

Yes, in nearly every case the cancellation terms arrive with the rate itself, so you can see them before your customer pays.

When your portal or your own system reads a hotel search result, each rate normally carries a policy telling you whether it can be cancelled free of charge, and if so, up to what date and time. Rates that cannot be cancelled are flagged as non refundable, and rates with a deadline will tell you what the charge becomes once that deadline has passed, which may be part of the stay or all of it depending on the hotel. The deadline is usually expressed in the hotel's local time, which matters when your customer is booking from another country, so it is worth showing the time zone next to the date.

How detailed this is does depend on the supplier behind the rate. Most send a clear, structured deadline that you can display to the customer and also use for your own internal reminders, while some send the terms as plain wording that reads well to a person but is harder to handle automatically. Terms can also differ between two rates at the very same hotel, because the policy belongs to the rate your customer selected rather than to the property as a whole. For that reason it is sensible to save the exact wording at the moment of booking, so you have a record of what was agreed if there is ever a disagreement about a refund.

Before you go live, run a few test searches across the different suppliers in your account and compare how the cancellation terms come back, then decide how you want them shown on your search results, your voucher and your booking confirmation.