If the tyres are not marked with a directional arrow or some reasonably obvious notation then they are not unidirectional. However it is often recommended that you do not change the direction of rotation once you have started to use them, i.e. keep the wheels on the same side of the vehicle and do not swap left to right. If you did not mark the wheels when you took them off to winterise, then may you have a slight problem!

Tyres with an asymetric left to right tread pattern must be bi-directional. These tyres should be marked inner or outer, but since they have to run on either side of the vehicle they must be designed to roll in either direction when the outside pattern faces outwards...