I heard a military leader (US) in the first days say that it couldn't have been a flying air to air missile and why. It had something to do with radar signatures
http://www.themalaymailonline.com/malaysia/article/how-the-us-knew-missile-shot-down-mh17
"How the US knew missile shot down MH17"
"An advanced spy satellite network sensitive enough to detect heat signatures from missiles
may have been used by the United States to conclude Flight MH17 was shot down over the Ukraine.
According to the Los Angeles Times (LAT), the specialised spy technology known as measurement
and signature intelligence (MASINT) allows the US to identify and track the electronic signatures
similarly to how radar operates."
http://www.latimes.com/nation/nationnow/la-fg-satellites-ukraine-missile-20140717-story.html
"High-tech spycraft tracked missile's path to Malaysia Airlines jet"
"The assessment was almost certainly based on a technical branch of spycraft known as measurement
and signature intelligence, or MASINT, analysts said. The method detects, tracks and identifies
a variety of electronic signatures, including radar."
"Separately, the U.S. operates sensitive listening satellites that can capture a wide range of
electronic emissions from foreign nations' defense systems, allowing analysts to determine the
origin of the signals and the type weapon that was used."