Short answer -- no, there is no easy way to know if a given image or text is copyrighted. The safest way to work is to assume everything is copyrighted unless you know
that it's public domain.
Copyright laws are a mess, and different for most different countries. (I am not a lawyer, but I do have a couple copyrighted short stories to my name.) There is an informal summary of U.S. copyright law <HERE>
. Sorry, it's a summary, but it's long. Copyright laws in other countries are different.
The bottom line is that anything that anyone puts to paper or other tangible form is covered by copyright unless they specifically release it to public domain, or work for an agency that produces for the public domain. That much is pretty much universal. The life of the of the copyright protection varies significantly, even for the same work in different countries -- like the U.S. and Canada. Here in the U.S., at least, the work does not have to bear a copyright notice or be registered with the copyright office to be copyrighted -- but it gains additional protection if it is registered with the Copyright office.
Copyright law is complicated by the fact that the law specifically permits "Fair Use" exceptions ... without explicitly defining what a Fair Use is. These exceptions are poorly defined until they get challenged but the most easily recognized (and defensible) would be quoting short sections, properly cited, for educational or other non-profit use. The guidelines are even less clear for images, but Wikipedia will occasionally use a copyrighted photo as a small, low-resolution thumbnail. But anything can be challenged.
As you noted, a ton of stuff flies over the internet that is technically copyrighted. And if you send one item to friends in different countries, it might be public domain in some and infringement in others.
Sorry, I know this isn't much help, but the laws here really are a mess!
Any lawyers are more than welcome to correct or clarify any part of my post.
Edited by Capn Easy, 03 March 2012 - 11:01 PM.