For PDF pages

With a page loaded in the Adobe Acrobat, on the File menu, click Properties, and then on the Fonts tab, read the list of fonts.

You may get more information about such fonts on Internet, e.g. in http://www.fontbureau.com/, and then typing the font name in the search box (top right).

In many situations you will use similar fonts, instead of original fonts.

For paper pages

It's all wild guessing - you need printed samples of various fonts, and then you have to try to recognize the font.