Unless the leak is glaring, you may not find it until you do as Kestas suggested and pressurize the system.
Auto Zone(and other mass marketers) rent a pressure tester for about $75.00. They let me keep it 3 days. You pay $75.00 when you rent it and they refund all $75.00 when you return it.
You perform this test on a cold engine. I had a bear of a leak awhile back on a 103 motor and would have never figured it out had I not rented this gadget.
You pull the radiator/expansion tank cap, connect an adapter if needed, then hook up the pressure tester. You pump it up by hand just like you would an old style tire pump.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.