Trying to play certain video files in different viewers (IrFanView for starters) and some won't open because a certain Codec is missing. Others with that same extension will play.
How do you determine what Codec is needed with a given file before you try to open it other than looking at the file extension?

I've had luck by dropping the video file into notepad and looking in the first few lines, the name of the codec should be visible there.

You could just hit it with a hammer and install the nimo codec pack - There's not a useful codec that exists that isn't included.

I've heard good things about gspot.
But I think it only works with .avi files, but then again those are the ones that seem to require the more obscure codecs.

Nimo is evil IMHO..
There is a program called avicodec that will tell you the vid and sound codec used in .avi and some .mpg files