How can I understand that the fonts in a PDF file are embedded or not?
In Adobe Reader, go to File -> Properties, then click on the Fonts tab. Embedded fonts will have either (Embedded) or (Embedded Subset) after the font name.
In Foxit Reader, the process is pretty similar. Go to File -> Properties, then click on the Fonts tab. Foxit doesn't say if it's a subset or not, but it does show (Embedded) after the font name if the font is embedded.
However, Foxit doesn't seem to identify all the embedded fonts -- looking at the Foxit manual (in PDF), it identifies a little over half of the embedded fonts as being embedded, but misses the rest. This is most likely a bug in Foxit Reader (version 3.2.1).
Look at the document properties inside the document (both Adobe Reader and FoxIt Reader include a property page which lists all fonts uses in the document and their status).