> On any specific technology, you absolutely can know that no one else has patents on it. That is the responsibility of the patent office: to grant exclusive rights to an 'invention'.
The problem though is that anything complex like a video or audio codec can constitute many "inventions" from the USPTO's point of view. H.264 has many patents on it, for example. Again, this is one of the big problems with the current patent system.
So in practical terms you can never use a codec with the knowledge that no one else has patents on it.