Winter tires break in I've just bought a pair of Kenda Klondike tires (26*2.1). I've heard that I have to ride about 50-100 kilometers without fast turns and so on - to make spikes to "sit" firmly into the tire.
So I've made my commute slowly.

Is that true? And if it is - how can I determine that this break in is done?
I work for a distributor that sells several brands of winter studded tires and everyone of them comes with this warning, this particular one is from Schwalbe


  In order to ensure that spikes are permanently fixed, tires should be run in for about 25 miles (40km) on asphalt, while avoiding any fast acceleration or heavy braking.


Source

With this said i don't think there is any physical means to tell that they have been ridden far enough. More so 25 miles/40km is likely on the safe side.

Here is the reason why they recommend the same for studded car tires and im quite certain the process is similar with bike tires.


  Looking like carpenter nails with their shafts cut short before being inserted headfirst into the tire, winter studs are made by encasing a tungsten carbide pin into a cylindrical metal housing. Typically 80 to 100 studs per tire are inserted into small holes molded in the tire's tread design. The tread is often lubricated (a 2% soapy water solution is desired) to facilitate installation. A special tool spreads the rubber and inserts the stud into the bottom of the hole. Once the stud is in place, the tool is removed and the tread rubber compresses around the stud's flat head and cylindrical housing to hold it in place.
  
  Because it takes some time for the lubricant to evaporate and the
  tread rubber to compress around the stud, studded tires require a
  special break-in period. This will allow the lubricant to
  evaporate and the tread rubber to conform to the shape of the stud.


Source

There is also a form of lubrication used when the tire are initially molded that remains on the surface of the tire for a period of time until worn off, this lubricant makes little difference to asphalt or dirt but as you can imagine on ice or snow it can make things a bit more slippery as it further reduces the friction coefficient.


  Before tires are cured, a release lubricant is often applied to prevent the tires from sticking in the mold. Unfortunately, some of the lubricant stays on the surface of the tires, and traction is reduced until it is worn away.