In many respects, the process of checking that your winter tyres are still in good condition does not differ too much from checking your summer or all season tyres. The difference lies in the fact that since you may only use these tyres for a few months a year, they could last a very long time before they need replacing.
You should check your winter tyres thoroughly every year before you put them back on your car for winter, to check that they will still do their job well. If they are not in good condition then there will be very little benefit in changing them over at all.