By how much is the salinity in the ocean increasing and how is this known?

The seas gain salt in a number of ways, the rivers taking dissolved salt from the rocks down to the sea, where it as to stay as no water runs out of the sea, and as the water in the oceans evaporates the salt is left behind and the ocean making the ocean saltier still, however this happens at an extremely slow rate and is like adding a few grains of salt to a pint of water every 100 years.
The seas are around 3% salty although this has been known to go as high as 3.08% to as low as 2.91% due to other factors. The sea will get saltier over time however there are also ways in which salt is removed from the oceans. All creatures need salt to live and they remove salt from the ocean this way, and when they die their bodies get locked up in the sedimentary rocks. Also the sea water reacts with sea rocks leaving behind salt this way.
Finally the massive plates that float on the Earth's mantle are constantly moving, when an ocean plate collides with a less dense continental plate then the continental plate will float over the top and the ocean plate will lose a large amount of its minerals deep into the earths crust.
Over recent years the salt in the sea has risen by a tiny fraction of a percentage every year, however scientists believe that this will slowly even its self out over hundreds and thousands of years.