Income Tax on salary income

Hi, I am an employer. I have hired one employee in the month of June 16 and I am paying him salary of Rs.7.7 Lacs p.a. I am also deducting TDS based on his investment declaration. During April to May 16 he was employed in other company and he has not declared me any income from his previous employment. My first question is that if incase he had earned some income around Rs.4.5 Lacs from previous employer & not declared to me who is responsible to pay tax on that assuming previous employer deducted nothing on that Rs.4.5 Lacs? Second question is that how income tax department can know that he (my employee) is evading income tax if he don't file his income tax return? If department come to know on this what will be amount of interest/penalty etc. & who is liable for that?