Sallyann12324.
I think most SAM systems are solid fuel based so the exhaust composition is different to jet or piston engine exhaust. If you know what the fuel is you can look for its characteristics.
Having a bit of a google showed up this article :
http://www.sbuv.com/MissileWarning/r&dproposal.html
which outlines how the detection technology works and avoids false alerts.