Sujit , can you tell us more ?
are we dealing with a flat sheet on a conveyer ?
is the plastic hot ?
How big is the area of this sheet , what thickness?
How fast is it moving ?
Often the simplest solution is a mechanical switch...
but ultrasonics could do the job , an optical system could be used if circumstances permit.. think refraction not reflection
Two other methods come to mind , first is the use of a theramin circuit
[google for "theramin vision"] this is a longshot but it might well work the other is to use a radioactive isotope coupled with a gieger detector ( often used in paper mills to measure and control thickness )