http://www.mtr.com.hk/eng/train/ae_intro.htm

Free In-town Check-in Service
To ensure that your journey gets off to a good start, a free In-town Check-in service is available for all major airlines at Hong Kong and Kowloon stations except:

Air Philippines
Angel Air
Continental Airlines
Continental Micronesia
Orient Thai
Pacific Airlines
Pakistan International
President Airlines
Turkish Airlines

With effective from 29 April 2003, passengers can check-in and collect their boarding passes from one day in advance to 100 minutes before take-off. (If you use the Octopus Card to ride on the Airport Express, fares are deducted when you check-in. But no more will be deducted when you board the train.)

The following airlines allow check-in one day prior to departure:

Aeroflot
Cathay Pacific Airways
China Airlines
Dragonair
Mandarin Airlines
Singapore Airlines*
South African Airlines
Swiss International Air Line
Thai Airways International PCL
United Airlines**
Vietnam Airlines

Service hours: 5:30am -11:00pm (The exact close-out time for individual airlines is subject to the flight schedule.)

* Due to the Security Policy, the In-town Check-in service of Singapore Airlines for flights flying to United States will be suspended temporarily.

** United Airlines only provides same day In-town Check-in service. The service hours are from 5:30am to 6:30pm.

More details, please contact respective airlines.