Thanks for above answers...

I am asked to put my past and current salary number on the background checking form. and the potential employer has asked me few times what my current salary is....I feel too that my current salary is not their business to know, while they just want to know... I feel they need to know it so they can add appropriate percentage on it to make me a offer.

so this is why I want to know if they are really able to check salary or not....