The Space Wolves have senses as acute as a Wolf of Fenris, superior even to other Space Marine Chapters. This is a result of their corrupt genetic implants and part of the blessing, as well as the curse, of the Wulfen. To represent this, you may re-roll the dice when rolling to see how far a Space Wolves unit can see when fighting at night. You must accept the result of the second roll, even if it is less than the first one.
Their acute senses make Space Wolves very difficult to sneak up on. To represent this, in scenarios which use the special rules for Sentries, a Space Wolves army uses four Grey Hunters as Sentries and each sentry has a spotting distance equal to twice its Initiative value.