In the US, photographs taken before 1963 and not registered and renewed, are now in the public domain. This includes the vast majority of amateur photography (maybe "all amateur photography," since registering w/the copyright office is a pretty clear mark of professional work), and most pictures published in newspapers and magazines.
Photos taken as late as 1980 are probably in the public domain *if* no proper copyright notice was ever attached to them. (I'm not sure how copyright notices get attached to photos. On the back? On the envelope? Unclear how that info would show up online--however, if you find pictures in a photo album and there's no copyright notice, those are PD.)
Photographs (and other works) created by gov't agencies are public domain. There's a weird line between "created by the government" (PD) and "created by a contractor hired by the government" (not always PD), but a bit of research can sort out which institutions have which kind of works.
The idea of "never use anything unless you can clear its copyright status first" is exactly the kind of chilling effect on creative works that the
Bound by Law comic was created to address.