How do I identify a missing codepage?

[TechKing]

I have nLited Windows XP to save space and removed most unused codepage/keyboards layout. I have installed Asian character support but kept only traditional and simplified Chinese. When I access a chinese webpage with Firefox or list filenames that contain chinese characters in explorer, boxes (sometime question marks in Firefox) instead of characters appear about 25% of the time. Obviously, one of the codepage has not been installed.

My question is: if you find a webpage that does not display the Asian characters appropriately, or a filename that does not display appropriately, how do I find out the exact codepage missing so I can install it manually?