Thanks Aparna, although your answer provided me quite good information but I asked that on which grounds/reasons we can say that the CMOS device/transistor is failed or rejectable.  

QA/Reliability characterization of transistors (MOSFET) is done under stress with rejection criteria based on many factors, including the reduction in drive strength at a given gate voltage, or a shift in the threshold voltage (from hot carrier and bias instability effects), gate leakage (as a precursor to gate oxide breakdown) are a few of them main ones.  Typically the transistors are stressed at increased voltage and temperature and I-V characteristics obtained from time-to-time during the stress are compared with the original (unstressed) I-V characertistics.