Is there a simple way of finding the font used in a piece of copied text?
Since you don't tell us which program you are copying from it will be difficult to give you a definitive answer, however...

In my tests with a PDF of my own creation preview.app does not seem to show font info in the info panel. However (the free) Adobe reader will show what fonts are used in the document if you go to File > Properties and select the Fonts tab.

That will show you what fonts are used in the document but not what font is at a specific point in the document.

There are web sites which purport to tell you what font is used from a picture (or screen shot) of the font in question. I remember trying this out a year or so ago with mixed results, but it might be worth searching for online.