If i have an invention and licence it to company A. Can company B find out. that I have a licence agrement signed with Company A?

First why would you want to hide the license agreement with company A from Company B? Sooner or later they would see the product on the shelf from a competitor. If you signed an exclusive license with a company you can't go to another company. If you signed a nonexclusive license then you can go to as many as will license it. Most companys want an exclusive to keep you from flooding the market with the product and hurting their sales potential.
Exclusive licenses normally pay more than nonexclusive for that reason.