May i know how the warhead in incoming missile can be identified whether it is conventional or nuclear ?
There is no knowledge of any system, at least in the public domain, that can detect and decipher a real nuclear warhead from a conventional warhead on an incoming missile. That is one of the prime intelligence failures of pre-emption.
With missiles that can travel around the world in about 20 mins. and SLBM's with flight times as short as 12 mins, pressure-packed timelines essentially reduce decision making to a series of checklists.
Heck, we even have problems distinguishing between real warheads and decoys. Even so, the problem essentially reduces down to detecting and destroying all real warheads <non-nuclear inclusive> and decoys, made of tin foil. Even with real and decoy warheads, radars find it difficult to distinguish between decoy and warhead because their spaces reflect radio waves similarly. Moreover, radars have to contend with clouds of thin metallic foil strips called chaff. Infrared devices, still for their part, have some difficulty distinguishing between warhead and decoys, but while tinfoil decoys quickly become cold in the surrounding space, warheads retain their heat and radiate it. So in theory, infrared devices of sufficient sophistication could, in tandem with anti-ballistic missiles, or sea-based, space-based, laser, and high altitude missile systems, detect and destroy real warheads <and thereby, nuclear warheads> in the post-boost and terminal phases. The SIBRIS (Space Based Infra Red System- US) due for launch in 2011, is intended to distinghuish between real and decoy warheads with a great degree of accuracy. The problem then, is ofcourse, how to minimize the fallout; ideally, you'd want to achieve it as high as possible, in the post-boost phase. And containment strategies, which could hypothetically be achieved by scattering radioactive-absorbent material in high low-earth orbit.