all (proper) winter tyres are marked M+S as this is the legal requirement for countries where they are obligatory.
good tyres that don't just bear the m+s mark for legal reasons (i.e. so you can drive your car all year round if legally winter tyres are called for) also have a snow flake symbol on them.
the 'm' can be interpreted as mud, but it's more snowy mud that really thick stuff you, e.g. find in a field.