How do military fighter aircraft identify themselves and each other? I live in an area where there is much NATO military activity. Often there are various fighter aircraft fly-overs. But none of the common websites such as flightradar24, flightaware and planeflighttracker etc. used for identifying aircraft using ADS-B and ACARS etc, are able to "see" them. 


So apart from ground radar, how does the military aircraft identify themselves?
Are there other type of transponders used in these?
Has there been any known attempts at building some amateur radar network to track these unidentified aircraft? 




There was little useful information in this SE answer. I also found after posting a very similar SE question and answers. However, it still doesn't fully address the questions.



EDIT: As an update for question (3), there seem to have been some success on using SDR's as passive radars, as shown in these links:



                
            

                
            
http://www.rtl-sdr.com/building-a-passive-radar-system-with-an-rtl-sdr/
http://www.rtl-sdr.com/tag/passive-radar/
http://www.kaminski.up.lublin.pl/RTL-SRD.html
http://hackaday.com/2015/06/05/building-your-own-sdr-based-passive-radar-on-a-shoestring/
Military aircraft, like civil aircraft have transponders. Military aircraft  are assigned military codes, and their transponders have special military modes.

An aircraft doesn't need a transponder to be "illuminated" by radar, but the transponder enhances the "blip" and modern transponders transmit additional aircraft information such as altitude (Mode C). 

See the "Interrogation Modes" of transponders in this link.

Notice that the Military modes include "cryptographically" secured version of Mode S and ADS-B GPS positioning.