Before leaving my home to go out spotting, I will always check to make sure that conditions are suitable for spotting. As you may know, “suitable” conditions for spotting is much more than just the absence of rain. The light coming from the sun must be bright and coming from the right direction. The visibility must be good, and it would be most preferable if the sky is blue.
I used to judge whether it was a good day to go spotting simply by looking outside the window in the morning and checking to make sure the weather is nice. Obviously this is a very ineffective and unreliable method for a number of reasons. First of all, conditions at the airport are always going to be very different in comparison to the weather where I live, which is around 30km away as the crow flies. The airport could be covered in pollution while my neighborhood is seeing clear blue skies (this actually happens quite often), or perhaps vice-versa. In addition, the looking-outside-the-window “method” won’t help you determine the direction of light, which is important when choosing a spotting location.
A much better source of information for spotting is your location airport webcam. Most major airports offer live-stream footage or photographs taken once every few minutes from security cameras on-site or at least not far from the airport. Although the image quality is often very poor, these cameras should give you plenty of information on current spotting conditions at the airport, more than what you can get from ATIS or by looking outside the window.
This website has links to webcam footage from airports all over the world, HKG included. Most major airports are included on the list, except for Singapore Changi and perhaps several others. It’s definitely a useful tool and will save you a lot frustration.
In addition, I recommend you take a look at your local Aerodome Forecast (TAF) to ensure that the weather will not change the worse when you’re out there spotting. Last but not least, don’t forget to check the civilian weather forecast for your city.