Technique where we can identify whether an employee is performing his/her job as per the job description, or is over burdened with it ?

There are certain employees who are performing tasks which are beyond their JD, and on the other hand there are employees who are not even performing the tasks which are listed out in their JD, so as an HR personnel is there any way or technique to identify such imbalance in the actual work and the JD.

Yes. you should draft HR policy which will include KRA's and their performance management system.
Performance of an employee will be measured perfectly with KRA.
Work smart

Hi,
You may introduce Daily Work Report concept so as to measure the work volume completed on daily basis.

Hi
The practical way is to check and measure the performance of your employees by establishing an evaluation. Such, should assess what are the purpose, expectation, and outcome subjectively and objectively of your organization.
Vlad