My employer is required to issue me a salary certificate in Form 16 every year if my salary exceeds taxable limits, or where my tax has been deducted on salary. So this will spell out what my salary is, and I just take this for the purpose of my tax return.
If somehow I am below the Form 16 eligibility, I can ask my employer to give me a certificate of what salary I got in the year.
At the beginning of the year, I need to be careful to tell my employer about any benefits I will be taking, such as on HRA (being on rent) , and my tax savings (see module later) so that my tax deduction from salary is planned appropriately. At year end, I need to give proofs of all such benefits or savings, else my employer will be constrained to deduct all tax at year end on concessions that I had planned which I have now failed to give proof of or am not eligible of.